Functional studies of the protein phosphatase-1 (PP1) regulator Sds22 suggest that it is indirectly and/or directly involved in one of the most ancient functions of PP1,i.e. reversing phosphorylation by the Aurora-related protein kinases. We predict that the conserved portion of Sds22 folds into a curved superhelix and demonstrate that mutation to alanine of any of eight residues (Asp148, Phe170, Glu192, Phe214, Asp280, Glu300, Trp302, or Tyr327) at the concave surface of this superhelix thwarts the interaction with PP1. Furthermore, we show that all mammalian isoforms of PP1 have the potential to bind Sds22. Interaction studies with truncated versions of PP1 and with chimeric proteins comprising fragments of PP1 and the yeast PP1-like protein phosphatase Ppz1 suggest that the site(s) required for the binding of Sds22 reside between residues 43 and 173 of PP1γ1. Within this region, a major interaction site was mapped to a triangular region delineated by the α4-, α5-, and α6-helices. Our data also show that well known regulatory binding sites of PP1, such as the RVXF-binding channel, the β12/β13-loop, and the acidic groove, are not essential for the interaction with Sds22. Functional studies of the protein phosphatase-1 (PP1) regulator Sds22 suggest that it is indirectly and/or directly involved in one of the most ancient functions of PP1,i.e. reversing phosphorylation by the Aurora-related protein kinases. We predict that the conserved portion of Sds22 folds into a curved superhelix and demonstrate that mutation to alanine of any of eight residues (Asp148, Phe170, Glu192, Phe214, Asp280, Glu300, Trp302, or Tyr327) at the concave surface of this superhelix thwarts the interaction with PP1. Furthermore, we show that all mammalian isoforms of PP1 have the potential to bind Sds22. Interaction studies with truncated versions of PP1 and with chimeric proteins comprising fragments of PP1 and the yeast PP1-like protein phosphatase Ppz1 suggest that the site(s) required for the binding of Sds22 reside between residues 43 and 173 of PP1γ1. Within this region, a major interaction site was mapped to a triangular region delineated by the α4-, α5-, and α6-helices. Our data also show that well known regulatory binding sites of PP1, such as the RVXF-binding channel, the β12/β13-loop, and the acidic groove, are not essential for the interaction with Sds22. Among the protein phosphatases that occur in all studied eukaryotic lineages, the Ser/Thr-specific protein phosphatases of type-1 are the best conserved, with more than 70% of their residues nearly invariant (1Ceulemans H. Stalmans W. Bollen M. BioEssays. 2002; 24: 371-381Google Scholar). This conservation extends well beyond structurally and catalytically important residues to include exposed residues involved in the binding of regulatory proteins. As a catalytic subunit, PP1 1The abbreviations used are: PP1, protein phosphatase-1; BLAST, basic local alignment search tool; CC, cysteine containing; EGFP, enhanced green fluorescent protein; LRR, leucine-rich repeat; RI, ribonuclease inhibitor; X-gal, 5-bromo-4-chloro-3-indolyl-β-d-galactoside 1The abbreviations used are: PP1, protein phosphatase-1; BLAST, basic local alignment search tool; CC, cysteine containing; EGFP, enhanced green fluorescent protein; LRR, leucine-rich repeat; RI, ribonuclease inhibitor; X-gal, 5-bromo-4-chloro-3-indolyl-β-d-galactoside depends on the interaction with one or two regulatory subunits for subcellular localization, substrate specificity, and activity regulation (1Ceulemans H. Stalmans W. Bollen M. BioEssays. 2002; 24: 371-381Google Scholar, 2Bollen M. Trends Biochem. Sci. 2001; 26: 426-431Google Scholar). Eukaryotic cells contain a large variety of regulatory subunits of PP1, which account for the diversified action of this phosphatase. We have recently proposed that PP1 acquired an essential function during early eukaryotic evolution by the development of sites for interaction with a primordial regulatory subunit(s) (1Ceulemans H. Stalmans W. Bollen M. BioEssays. 2002; 24: 371-381Google Scholar). This essential primordial function and the sequential acquirement of additional interaction sites and functions would then have impeded further mutation of the corresponding portion(s) of the surface. The phylogenetic distribution of PP1 indicates that this primordial function must have been acquired before the divergence of the extant eukaryotic lineages. One of the most ancient functions of PP1 is to dephosphorylate substrates of the Aurora-related protein kinases, and this is essential for the completion of mitosis (3Hsu J.Y. Sun Z.W. Li X. Reuben M. Tatchell K. Bishop D.K. Gruschow J.M. Brame C.J. Caldwell J.A. Hunt D.F. Lin R. Smith M.M. Allis C.D. Cell. 2000; 102: 279-291Google Scholar). The regulatory subunit(s) associated with this function of PP1 remain unknown, but the protein Sds22 (38 kDa) has emerged as a prime candidate. First, both yeast and mammalian Sds22 have been shown to interact with PP1 and to be part of a complex with PP1 that is enriched in the nucleus (4Hisamoto N. Frederick D.L. Sugimoto K. Tatchell K. Matsumoto K. Mol. Cell. Biol. 1995; 15: 3767-3776Google Scholar, 5MacKelvie S.H. Andrews P.D. Stark M.J. Mol. Cell. Biol. 1995; 15: 3777-3785Google Scholar, 6Stone E.M. Yamano H. Kinoshita N. Yanagida M. Curr. Biol. 1993; 3: 13-26Google Scholar, 7Dinischiotu A. Beullens M. Stalmans W. Bollen M. FEBS Lett. 1997; 402: 141-144Google Scholar). Second, the Sds22 encoding gene was identified independently in fission and in budding yeast as an extra-copy suppressor of the temperature-sensitive mitotic arrest phenotypes that are associated with certain mutations of PP1 (4Hisamoto N. Frederick D.L. Sugimoto K. Tatchell K. Matsumoto K. Mol. Cell. Biol. 1995; 15: 3767-3776Google Scholar, 5MacKelvie S.H. Andrews P.D. Stark M.J. Mol. Cell. Biol. 1995; 15: 3777-3785Google Scholar, 8Ohkura H. Yanagida M. Cell. 1991; 57: 997-1007Google Scholar). Deletion of the Sds22-encoding gene caused a similar mitotic arrest, and this phenotype could be complemented by the overexpression of PP1 (4Hisamoto N. Frederick D.L. Sugimoto K. Tatchell K. Matsumoto K. Mol. Cell. Biol. 1995; 15: 3767-3776Google Scholar, 5MacKelvie S.H. Andrews P.D. Stark M.J. Mol. Cell. Biol. 1995; 15: 3777-3785Google Scholar, 8Ohkura H. Yanagida M. Cell. 1991; 57: 997-1007Google Scholar). Third, the conditionally lethal phenotype in budding yeast that was conferred by a loss-of-function mutation of the Aurora-related kinase Ipl1 (Ipl1–2), was largely relieved by the expression of certain temperature-sensitive mutant versions of Sds22 or PP1 (9Peggie M.W. MacKelvie S.H. Blocher A. Knatko E.V. Tatchell K. Stark M.J. J. Cell Sci. 2002; 115: 195-206Google Scholar, 10Francisco L. Wang W. Chan C.S. Mol. Cell. Biol. 1994; 14: 4731-4740Google Scholar). The mutant Sds22 version that rescued the Ipl1–2 phenotype showed a decreased ability to interact with PP1. The expression of this mutant Sds22 did not affect the cellular levels of PP1 or Sds22, but drastically reduced the nuclear level of PP1 and caused a redistribution of the nuclear pool of PP1 (9Peggie M.W. MacKelvie S.H. Blocher A. Knatko E.V. Tatchell K. Stark M.J. J. Cell Sci. 2002; 115: 195-206Google Scholar).Hitherto, little is known about the mechanism of interaction between Sds22 and PP1. Most known PP1 regulators contain a so-called “RVXF” motif with the consensus sequence [RK]X (0–1)[VI]X[FW], which binds to a hydrophobic channel of the catalytic subunit (1Ceulemans H. Stalmans W. Bollen M. BioEssays. 2002; 24: 371-381Google Scholar, 11Egloff M.P. Johnson D.F. Moorhead G. Cohen P.T. Cohen P. Barford D. EMBO J. 1997; 16: 1876-1887Google Scholar). Sds22 lacks an RVXF motif but consists of a tandem array of leucine-rich repeats (LRRs), which are established protein interaction modules (12Kobe B. Kajava A.V. Curr. Opin. Struct. Biol. 2001; 11: 725-732Google Scholar). In this paper, we demonstrate that the LRR-repeats of Sds22 are indeed essential for binding to PP1 and we propose that the LRRs assume the conformation of a curved superhelix ending in a C-terminal so-called LRR cap (13Ceulemans H. De Maeyer M. Stalmans W. Bollen M. FEBS Lett. 1999; 456: 349-351Google Scholar). Guided by this three-dimensional model and by the crystal structure of PP1, we have been able to map determinants of the Sds22-PP1 interaction at the concave surface of the Sds22 superhelix and in a triangle composed of α-helices 4, 5, and 6 of PP1.DISCUSSIONThe present study established that the conserved C-terminal three-fourths of Sds22, which are proposed to form a curved LRR superhelix fused to a C-terminal LRR-cap, suffice for binding to PP1. Nevertheless, the affinity of Sds22 for PP1 was significantly reduced by removal of the N terminus of Sds22. Strikingly, a similar N-terminal amputation of fission yeast Sds22 confers a temperature-sensitive mitotic defect (6Stone E.M. Yamano H. Kinoshita N. Yanagida M. Curr. Biol. 1993; 3: 13-26Google Scholar), which may also correlate with compromised binding to PP1. Possibly, the N terminus of Sds22 folds into an N-terminal LRR-cap, like the one observed in the structure of Internalin B, a prokaryotic LRR protein of the Sds22-like family (38Schubert W.D. Gobel G. Diepholz M. Darji A. Kloer D. Hain T. Chakraborty T. Wehland J. Domann J. Mol. Biol. 2001; Scholar). Deletion of such a cap may a of of the LRRs to of the hydrophobic of the a major interaction site for PP1, comprising residues Phe170, Glu192, Phe214, Asp280, Glu300, Trp302, and of Sds22, was mapped to the concave surface of the mutation of any of residues or compromised the interaction with PP1. The of a of yeast was in mammalian in the crystal of LRR proteins in complex with the concave of the also functions as a binding site K. Scholar, M.M. Sci. 2002; Scholar, J. Mol. Biol. 2001; B. J. 1995; the concave surface be in of and the proposed of residues also the of the be that in of the of the we have for a as observed in eukaryotic proteins with Sds22-like LRRs and an LRR-cap, a of the be a in the proteins Internalin B, which consists largely of Sds22-like LRRs (38Schubert W.D. Gobel G. Diepholz M. Darji A. Kloer D. Hain T. Chakraborty T. Wehland J. Domann J. Mol. Biol. 2001; Scholar), and which has LRRs of the so-called R. EMBO J. 1997; 16: Scholar). The of proteins has been by the of exposed that a in the of repeats A.V. B. Sci. 2002; 11: Scholar). The in the consensus sequence of the Sds22 repeats may a similar have also that Sds22 bind all mammalian isoforms of PP1. that most of the between isoforms is in the N-terminal and C-terminal this is in with that the N terminus of PP1 not function as a binding site for Sds22. Nevertheless, removal of the N terminus to the of a of the truncated and studies and with chimeric proteins suggest that the essential sites are between residues of which interaction sites such as the RVXF-binding channel, the β12/β13-loop, and the acidic as essential determinants of the In with it has recently been shown that mutations in the RVXF-binding channel that impeded binding of PP1 regulators did not the interaction with Sds22 X. Tatchell K. 2001; Scholar). In for we the that a interaction in yeast than did the was with this be to the of the terminus of PP1 but is indeed caused by the of residues that include the that RVXF-binding is by this it is that the binding of Sds22 to this truncated version of PP1 is by the of with subunits that are with Sds22 it be that of residues of the Sds22-PP1 that this an binding site for Sds22 is such a site would and the crystal structure of PP1 M.P. Johnson D.F. Moorhead G. Cohen P.T. Cohen P. Barford D. EMBO J. 1997; 16: 1876-1887Google that must have been in and which a enhanced interaction the mutations in Sds22 that the binding to also binding to that the C-terminal of PP1, all residues that to the RVXF-binding channel, is not required for the interaction with Sds22 the that in PP1 this channel is for interaction with a additional the of that with Sds22 and PP1 fission yeast (6Stone E.M. Yamano H. Kinoshita N. Yanagida M. Curr. Biol. 1993; 3: 13-26Google Scholar), may a of the Sds22 PP1 are known that contain an and an such as the complex L. M. M. T. J. also that PP1 largely lacks phosphatase activity A. Beullens M. Stalmans W. Bollen M. FEBS Lett. 1997; 402: 141-144Google Scholar, G. E.M. Biochem. 2000; Scholar). not that Sds22 is a regulator of PP1 the of regulators is well known to be M. Trends Biochem. Sci. 2001; 26: 426-431Google Scholar). indeed be that the binding of Sds22 to the of PP1 the of the catalytic site to and PP1 such as we have into the complex interaction mechanism that the binding of Sds22 to PP1 and have major binding sites on both interaction Among the protein phosphatases that occur in all studied eukaryotic lineages, the Ser/Thr-specific protein phosphatases of type-1 are the best conserved, with more than 70% of their residues nearly invariant (1Ceulemans H. Stalmans W. Bollen M. BioEssays. 2002; 24: 371-381Google Scholar). This conservation extends well beyond structurally and catalytically important residues to include exposed residues involved in the binding of regulatory proteins. As a catalytic subunit, PP1 1The abbreviations used are: PP1, protein phosphatase-1; BLAST, basic local alignment search tool; CC, cysteine containing; EGFP, enhanced green fluorescent protein; LRR, leucine-rich repeat; RI, ribonuclease inhibitor; X-gal, 5-bromo-4-chloro-3-indolyl-β-d-galactoside 1The abbreviations used are: PP1, protein phosphatase-1; BLAST, basic local alignment search tool; CC, cysteine containing; EGFP, enhanced green fluorescent protein; LRR, leucine-rich repeat; RI, ribonuclease inhibitor; X-gal, 5-bromo-4-chloro-3-indolyl-β-d-galactoside depends on the interaction with one or two regulatory subunits for subcellular localization, substrate specificity, and activity regulation (1Ceulemans H. Stalmans W. Bollen M. BioEssays. 2002; 24: 371-381Google Scholar, 2Bollen M. Trends Biochem. Sci. 2001; 26: 426-431Google Scholar). Eukaryotic cells contain a large variety of regulatory subunits of PP1, which account for the diversified action of this phosphatase. We have recently proposed that PP1 acquired an essential function during early eukaryotic evolution by the development of sites for interaction with a primordial regulatory subunit(s) (1Ceulemans H. Stalmans W. Bollen M. BioEssays. 2002; 24: 371-381Google Scholar). This essential primordial function and the sequential acquirement of additional interaction sites and functions would then have impeded further mutation of the corresponding portion(s) of the surface. The phylogenetic distribution of PP1 indicates that this primordial function must have been acquired before the divergence of the extant eukaryotic lineages. One of the most ancient functions of PP1 is to dephosphorylate substrates of the Aurora-related protein kinases, and this is essential for the completion of mitosis (3Hsu J.Y. Sun Z.W. Li X. Reuben M. Tatchell K. Bishop D.K. Gruschow J.M. Brame C.J. Caldwell J.A. Hunt D.F. Lin R. Smith M.M. Allis C.D. Cell. 2000; 102: 279-291Google Scholar). The regulatory subunit(s) associated with this function of PP1 remain unknown, but the protein Sds22 (38 kDa) has emerged as a prime candidate. First, both yeast and mammalian Sds22 have been shown to interact with PP1 and to be part of a complex with PP1 that is enriched in the nucleus (4Hisamoto N. Frederick D.L. Sugimoto K. Tatchell K. Matsumoto K. Mol. Cell. Biol. 1995; 15: 3767-3776Google Scholar, 5MacKelvie S.H. Andrews P.D. Stark M.J. Mol. Cell. Biol. 1995; 15: 3777-3785Google Scholar, 6Stone E.M. Yamano H. Kinoshita N. Yanagida M. Curr. Biol. 1993; 3: 13-26Google Scholar, 7Dinischiotu A. Beullens M. Stalmans W. Bollen M. FEBS Lett. 1997; 402: 141-144Google Scholar). Second, the Sds22 encoding gene was identified independently in fission and in budding yeast as an extra-copy suppressor of the temperature-sensitive mitotic arrest phenotypes that are associated with certain mutations of PP1 (4Hisamoto N. Frederick D.L. Sugimoto K. Tatchell K. Matsumoto K. Mol. Cell. Biol. 1995; 15: 3767-3776Google Scholar, 5MacKelvie S.H. Andrews P.D. Stark M.J. Mol. Cell. Biol. 1995; 15: 3777-3785Google Scholar, 8Ohkura H. Yanagida M. Cell. 1991; 57: 997-1007Google Scholar). Deletion of the Sds22-encoding gene caused a similar mitotic arrest, and this phenotype could be complemented by the overexpression of PP1 (4Hisamoto N. Frederick D.L. Sugimoto K. Tatchell K. Matsumoto K. Mol. Cell. Biol. 1995; 15: 3767-3776Google Scholar, 5MacKelvie S.H. Andrews P.D. Stark M.J. Mol. Cell. Biol. 1995; 15: 3777-3785Google Scholar, 8Ohkura H. Yanagida M. Cell. 1991; 57: 997-1007Google Scholar). Third, the conditionally lethal phenotype in budding yeast that was conferred by a loss-of-function mutation of the Aurora-related kinase Ipl1 (Ipl1–2), was largely relieved by the expression of certain temperature-sensitive mutant versions of Sds22 or PP1 (9Peggie M.W. MacKelvie S.H. Blocher A. Knatko E.V. Tatchell K. Stark M.J. J. Cell Sci. 2002; 115: 195-206Google Scholar, 10Francisco L. Wang W. Chan C.S. Mol. Cell. Biol. 1994; 14: 4731-4740Google Scholar). The mutant Sds22 version that rescued the Ipl1–2 phenotype showed a decreased ability to interact with PP1. The expression of this mutant Sds22 did not affect the cellular levels of PP1 or Sds22, but drastically reduced the nuclear level of PP1 and caused a redistribution of the nuclear pool of PP1 (9Peggie M.W. MacKelvie S.H. Blocher A. Knatko E.V. Tatchell K. Stark M.J. J. Cell Sci. 2002; 115: 195-206Google Scholar). little is known about the mechanism of interaction between Sds22 and PP1. Most known PP1 regulators contain a so-called “RVXF” motif with the consensus sequence [RK]X (0–1)[VI]X[FW], which binds to a hydrophobic channel of the catalytic subunit (1Ceulemans H. Stalmans W. Bollen M. BioEssays. 2002; 24: 371-381Google Scholar, 11Egloff M.P. Johnson D.F. Moorhead G. Cohen P.T. Cohen P. Barford D. EMBO J. 1997; 16: 1876-1887Google Scholar). Sds22 lacks an RVXF motif but consists of a tandem array of leucine-rich repeats (LRRs), which are established protein interaction modules (12Kobe B. Kajava A.V. Curr. Opin. Struct. Biol. 2001; 11: 725-732Google Scholar). In this paper, we demonstrate that the LRR-repeats of Sds22 are indeed essential for binding to PP1 and we propose that the LRRs assume the conformation of a curved superhelix ending in a C-terminal so-called LRR cap (13Ceulemans H. De Maeyer M. Stalmans W. Bollen M. FEBS Lett. 1999; 456: 349-351Google Scholar). Guided by this three-dimensional model and by the crystal structure of PP1, we have been able to map determinants of the Sds22-PP1 interaction at the concave surface of the Sds22 superhelix and in a triangle composed of α-helices 4, 5, and 6 of PP1. present study established that the conserved C-terminal three-fourths of Sds22, which are proposed to form a curved LRR superhelix fused to a C-terminal LRR-cap, suffice for binding to PP1. Nevertheless, the affinity of Sds22 for PP1 was significantly reduced by removal of the N terminus of Sds22. Strikingly, a similar N-terminal amputation of fission yeast Sds22 confers a temperature-sensitive mitotic defect (6Stone E.M. Yamano H. Kinoshita N. Yanagida M. Curr. Biol. 1993; 3: 13-26Google Scholar), which may also correlate with compromised binding to PP1. Possibly, the N terminus of Sds22 folds into an N-terminal LRR-cap, like the one observed in the structure of Internalin B, a prokaryotic LRR protein of the Sds22-like family (38Schubert W.D. Gobel G. Diepholz M. Darji A. Kloer D. Hain T. Chakraborty T. Wehland J. Domann J. Mol. Biol. 2001; Scholar). Deletion of such a cap may a of of the LRRs to of the hydrophobic of the a major interaction site for PP1, comprising residues Phe170, Glu192, Phe214, Asp280, Glu300, Trp302, and of Sds22, was mapped to the concave surface of the mutation of any of residues or compromised the interaction with PP1. The of a of yeast was in mammalian in the crystal of LRR proteins in complex with the concave of the also functions as a binding site K. Scholar, M.M. Sci. 2002; Scholar, J. Mol. Biol. 2001; B. J. 1995; the concave surface be in of and the proposed of residues also the of the be that in of the of the we have for a as observed in eukaryotic proteins with Sds22-like LRRs and an LRR-cap, a of the be a in the proteins Internalin B, which consists largely of Sds22-like LRRs (38Schubert W.D. Gobel G. Diepholz M. Darji A. Kloer D. Hain T. Chakraborty T. Wehland J. Domann J. Mol. Biol. 2001; Scholar), and which has LRRs of the so-called R. EMBO J. 1997; 16: Scholar). The of proteins has been by the of exposed that a in the of repeats A.V. B. Sci. 2002; 11: Scholar). The in the consensus sequence of the Sds22 repeats may a similar have also that Sds22 bind all mammalian isoforms of PP1. that most of the between isoforms is in the N-terminal and C-terminal this is in with that the N terminus of PP1 not function as a binding site for Sds22. Nevertheless, removal of the N terminus to the of a of the truncated and studies and with chimeric proteins suggest that the essential sites are between residues of which interaction sites such as the RVXF-binding channel, the β12/β13-loop, and the acidic as essential determinants of the In with it has recently been shown that mutations in the RVXF-binding channel that impeded binding of PP1 regulators did not the interaction with Sds22 X. Tatchell K. 2001; Scholar). In for we the that a interaction in yeast than did the was with this be to the of the terminus of PP1 but is indeed caused by the of residues that include the that RVXF-binding is by this it is that the binding of Sds22 to this truncated version of PP1 is by the of with subunits that are with Sds22 it be that of residues of the Sds22-PP1 that this an binding site for Sds22 is such a site would and the crystal structure of PP1 M.P. Johnson D.F. Moorhead G. Cohen P.T. Cohen P. Barford D. EMBO J. 1997; 16: 1876-1887Google that must have been in and which a enhanced interaction the mutations in Sds22 that the binding to also binding to that the C-terminal of PP1, all residues that to the RVXF-binding channel, is not required for the interaction with Sds22 the that in PP1 this channel is for interaction with a additional the of that with Sds22 and PP1 fission yeast (6Stone E.M. Yamano H. Kinoshita N. Yanagida M. Curr. Biol. 1993; 3: 13-26Google Scholar), may a of the Sds22 PP1 are known that contain an and an such as the complex L. M. M. T. J. also that PP1 largely lacks phosphatase activity A. Beullens M. Stalmans W. Bollen M. FEBS Lett. 1997; 402: 141-144Google Scholar, G. E.M. Biochem. 2000; Scholar). not that Sds22 is a regulator of PP1 the of regulators is well known to be M. Trends Biochem. Sci. 2001; 26: 426-431Google Scholar). indeed be that the binding of Sds22 to the of PP1 the of the catalytic site to and PP1 such as we have into the complex interaction mechanism that the binding of Sds22 to PP1 and have major binding sites on both interaction The present study established that the conserved C-terminal three-fourths of Sds22, which are proposed to form a curved LRR superhelix fused to a C-terminal LRR-cap, suffice for binding to PP1. Nevertheless, the affinity of Sds22 for PP1 was significantly reduced by removal of the N terminus of Sds22. Strikingly, a similar N-terminal amputation of fission yeast Sds22 confers a temperature-sensitive mitotic defect (6Stone E.M. Yamano H. Kinoshita N. Yanagida M. Curr. Biol. 1993; 3: 13-26Google Scholar), which may also correlate with compromised binding to PP1. Possibly, the N terminus of Sds22 folds into an N-terminal LRR-cap, like the one observed in the structure of Internalin B, a prokaryotic LRR protein of the Sds22-like family (38Schubert W.D. Gobel G. Diepholz M. Darji A. Kloer D. Hain T. Chakraborty T. Wehland J. Domann J. Mol. Biol. 2001; Scholar). Deletion of such a cap may a of of the LRRs to of the hydrophobic of the Furthermore, a major interaction site for PP1, comprising residues Phe170, Glu192, Phe214, Asp280, Glu300, Trp302, and of Sds22, was mapped to the concave surface of the mutation of any of residues or compromised the interaction with PP1. The of a of yeast was in mammalian in the crystal of LRR proteins in complex with the concave of the also functions as a binding site K. Scholar, M.M. Sci. 2002; Scholar, J. Mol. Biol. 2001; B. J. 1995; Scholar). the concave surface be in of and the proposed of residues also the of the be that in of the of the we have for a as observed in eukaryotic proteins with Sds22-like LRRs and an LRR-cap, a of the be a in the proteins Internalin B, which consists largely of Sds22-like LRRs (38Schubert W.D. Gobel G. Diepholz M. Darji A. Kloer D. Hain T. Chakraborty T. Wehland J. Domann J. Mol. Biol. 2001; Scholar), and which has LRRs of the so-called R. EMBO J. 1997; 16: Scholar). The of proteins has been by the of exposed that a in the of repeats A.V. B. Sci. 2002; 11: Scholar). The in the consensus sequence of the Sds22 repeats may a similar We have also that Sds22 bind all mammalian isoforms of PP1. that most of the between isoforms is in the N-terminal and C-terminal this is in with that the N terminus of PP1 not function as a binding site for Sds22. Nevertheless, removal of the N terminus to the of a of the truncated and studies and with chimeric proteins suggest that the essential sites are between residues of which interaction sites such as the RVXF-binding channel, the β12/β13-loop, and the acidic as essential determinants of the In with it has recently been shown that mutations in the RVXF-binding channel that impeded binding of PP1 regulators did not the interaction with Sds22 X. Tatchell K. 2001; Scholar). In for we the that a interaction in yeast than did the was with this be to the of the terminus of PP1 but is indeed caused by the of residues that include the that RVXF-binding is by this it is that the binding of Sds22 to this truncated version of PP1 is by the of with subunits that are with Sds22 it be that of residues of the Sds22-PP1 that this an binding site for Sds22 is such a site would and the crystal structure of PP1 M.P. Johnson D.F. Moorhead G. Cohen P.T. Cohen P. Barford D. EMBO J. 1997; 16: 1876-1887Google that must have been in and which a enhanced interaction the mutations in Sds22 that the binding to also binding to PP1γ1. The that the C-terminal of PP1, all residues that to the RVXF-binding channel, is not required for the interaction with Sds22 the that in PP1 this channel is for interaction with a additional the of that with Sds22 and PP1 fission yeast (6Stone E.M. Yamano H. Kinoshita N. Yanagida M. Curr. Biol. 1993; 3: 13-26Google Scholar), may a of the Sds22 PP1 are known that contain an and an such as the complex L. M. M. T. J. Scholar). We also that PP1 largely lacks phosphatase activity A. Beullens M. Stalmans W. Bollen M. FEBS Lett. 1997; 402: 141-144Google Scholar, G. E.M. Biochem. 2000; Scholar). not that Sds22 is a regulator of PP1 the of regulators is well known to be M. Trends Biochem. Sci. 2001; 26: 426-431Google Scholar). indeed be that the binding of Sds22 to the of PP1 the of the catalytic site to and PP1 such as In we have into the complex interaction mechanism that the binding of Sds22 to PP1 and have major binding sites on both interaction is for the of the and Beullens for the of and
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