The Endoplasmic Reticulum—Gateway of the Secretory Pathway

In the mid to late 1960s, the discovery that eukaryotic secreted proteins are first segregated in the lumen of the endoplasmic reticulum (ER) before traveling within membranous structures to reach the cell surface placed the ER at the start point of a newly recognized metabolic pathway now known as the secretory pathway (Palade, 1975). In a review based mainly on observations of plant cells, Morré and Mollenhauer (1974) posited the endomembrane system as the functional integration of the ER, Golgi complex, secretory vesicles, plasma membrane, and hydrolytic compartments (vacuoles in plants and lysosomes in animals). A number of intermediate compartments have been identified more recently (see Battey et al., 1999; Marty, 1999; Sanderfoot and Raikhel, 1999, in this issue). In the secretory pathway, proteins travel from the ER through the Golgi apparatus to arrive at the cell surface or at vacuoles. This biosynthetic, or anterograde, traffic is balanced by retrograde traffic running in the opposite direction. The retrograde traffic allows for endocytosis of extracellular molecules as well as recycling of membranes and proteins to maintain the integrity of the different compartments. Routes from the ER to vacuoles, bypassing the Golgi complex, also exist (Levanony et al., 1992; Robinson et al., 1995; Hara-Nishimura et al., 1998; Jiang and Rogers, 1998; see Sanderfoot and Raikhel, 1999, in this issue). Protein trafficking in the secretory pathway can occur both via vesicle budding with subsequent fusion and via passage along connecting tubules. The lumens of the ER, Golgi complex, and vacuole are topologically equivalent to the cell exterior, and extracellular domains of plasma membrane proteins are thus lumenal when they start their journey in the ER. Because nearly all eukaryotic proteins are synthesized in the cytosol, those destined to reside in a different location must cross membranes in an unfolded state and then fold and assemble in the compartment of destination. However, the mechanisms of traffic along the secretory pathway involve only a single translocation event across the ER membrane, after which proteins do not have to cross any further membrane to reach other stations along the pathway. The ER is therefore unique among the compartments of the eukaryotic cell. It takes care of the folding and assembly not only of its own residents but also of proteins destined to other locations. The crucial role of the ER in safeguarding the correct folding and assembly of proteins has become clearer in the past two decades with the discovery of the ER molecular chaperone machinery and the associated quality control mechanisms (the term “quality control” was introduced in a review by Hurtley and Helenius, 1989). These safeguards not only optimize folding and assembly of newly synthesized secretory proteins but also dispose of defective ones (Haas and Wabl, 1983; Munro and Pelham, 1986; Hammond et al., 1994; Wiertz et al., 1996; Pedrazzini et al., 1997). Secretory proteins usually enter the ER cotranslationally. Translocation of soluble proteins depends on the N-terminal signal peptide. Integral membrane proteins may also be synthesized with an N-terminal signal peptide, but their orientation within the ER membrane and the translocation of portions that will eventually reside in the lumen are determined by the corresponding membrane-spanning domains. The N-terminal signal peptide is removed cotranslationally while the nascent polypeptide is emerging into the ER lumen (Vitale et al., 1993). Removal is performed by signal peptidase, an enzyme located on the lumenal surface of the ER membrane. For many years, translocation across the ER membrane was thought to occur directly through the membrane itself by virtue of the hydrophobic core of the signal peptide. Studies with fluorescently labeled proteins, however, have shown that polypeptides remain in an aqueous environment during translocation (Crowley et al., 1993). Thus, the current model of protein translocation involves a multiprotein pore, termed the translocon pore, with an aqueous channel of 9 to 15 Å in diameter when inactive and 40 to 60 Å when operational (Hamman et al., 1998). The growing polypeptide emerges on the lumenal side of the membrane and to fold in the ER the protein a as do many membrane proteins, then translocation is and the hydrophobic into the membrane to become domains. The current model not for protein translocation but the as to the of the ER is in of the in its membrane. however, a to for the of the ER membrane. in the lumenal protein the lumenal of the inactive when is in protein the is on the side by the and is from the lumenal side (Crowley et al., et al., 1998). The of the ER which from that of the cytosol, is thus at all and molecular that reside in the ER have been identified and have been termed are in compartment in which protein folding and assembly occur in to proteins that are to molecular to of and assembly and the of side that to Thus, molecular the but not the of correct as of folding It is that protein folding in Translocation across the ER of the ER membrane an translocon and an operational translocon the translocon at the lumenal a to the translocation pore, the is and the the as to maintain the integrity of the ER The nascent polypeptide through the aqueous channel at the of the translocon into the to by are different of molecular the the protein the and the et al., The that are not in the ER, at in this The other have ER as and which to the and and which are the of the ER as have been only in are et al., The only ER chaperone for which the of has been in is which in for a of are to be on the of folding and assembly as well as proteins, the by in is by the and of folding of the polypeptide et the other is an and its protein in the of and Pelham, 1986; In protein folding in the ER is by folding as protein which the of and the which with nascent The two are to the ER and of have been The of the signal peptide is thought to be to the correct folding of the N-terminal of the growing The that a signal peptide can is not removed can be in the of a of that the of the proteins of this are that in the ER of as protein that to in and 1999, in this issue). In a has a point at the signal peptide that an is by a et al., a the signal peptide is not and the to the ER membrane et al., 1995; et al., 1997). The of this is In protein have a of all to and the of ER molecular and is These are in plants with the the et al., 1997). the fold a and on the protein assembly It is not known the in is to the of translocon or to quality control of protein The of and in plants et al., and a role for proteins during ER in of proteins or in proteins for of the that protein folding and assembly within the ER have been determined in within compartments of a of in the ER which is to the et al., 1998). However, in the cytosol, the of to the ER an environment that et al., that protein folding in the ER metabolic of the of in the of et al., of with the in the protein and its that the in state of the plant ER is for the correct of and on the assembly of the proteins of termed into the and for their folding and its folding in the ER of cells, two and then into which are to the protein The of can be in an in system in which a that a polypeptide the signal peptide is et al., of to the however, the ER lumen and the of that the of the ER assembly et al., 1997). This that the of is for the folding of into the for assembly et al., 1997). of in the in system can be by an enzyme et al., 1997). The that the to a role of molecular in the the of in the folding and of secretory proteins are at in the is any but In to the have to be to of the on the Thus, may be or not at from with a with the (Vitale et al., 1993). The enzyme on the lumenal side of translocon pore, the from a in the ER membrane to of nascent polypeptides as they enter the ER may also occur (Vitale et al., 1993). is that as a of protein folding in the ER. of the of are removed in the ER by two which the and which the two (Vitale et al., 1993). can the to the of among the first in which the of in the ER was et al., The of this the discovery of the of the and the of the of two membrane and soluble These and the the of those that are not et al., 1997). are not recognized by the and are therefore from this The thus the of protein folding and to and polypeptides within the folding environment of the ER. for a system in plants the that is associated with a of newly synthesized et al., may be In is by a to protein folding et al., for the role of in protein folding has been for is not for and of (Vitale et al., but the of assembly of polypeptides is to the number of (Vitale et al., 1995; et al., 1997). In the of assembly of is when by ER is to the with et al., 1997). This the that the machinery is not a of folding or assembly but that as a of is by the of the for and is et al., 1995; Pedrazzini and with to and of in the ER and and eventually to the of and These that at must their for correct of on cell may thus be to the of of and the may directly hydrophobic peptide to the protein surface during The ER by then be to to correct The in the to of the that the proteins of the folding of a however, to be The of in the of plant secretory proteins has been directly in proteins assemble into their in the ER et al., before via the Golgi to the in with (the protein of in an not with that from the ER (Vitale et al., Thus, only as The newly synthesized of other proteins, and are also in with The of newly synthesized protein that is to is protein different of in the ER (Vitale et al., of protein that in the ER. with nascent emerging into the ER lumen when are to the and is also at the surface of protein but not et al., et al., 1997). thus from their to their assembly into protein when the polypeptides are into the protein are their into protein both in the ER and in (see and 1999, in this issue). protein can from that reach the via the Golgi complex, or they from the of protein in the ER (Levanony et al., is the compartment et al., It is not known this of within the protein during their in the ER or an role of the chaperone at their The is a membrane protein of a to the of to different and an of two to a review of membrane see et al., 1999, in this issue). from are with for the complex, the is along with et al., 1998). of is These that in the assembly of this membrane protein and that the with its membrane when the is in the ER. For the in or in have on the of with a of However, was recently shown that the of secretory protein et al., 1998). in in plant cells, a functional was the secretory protein and a enzyme as a control for cell an of was when with the the is not and can therefore not the enzyme The was shown to be to the of in the ER, is to et al., The is on the protein as well as on the of which that is in the of protein on the ER. This is a to in secretory protein in which can now be to of by The of quality control in the ER two of protein and Helenius, proteins that are recognized as or as folding or defective proteins by or be within the ER. A be the of proteins for control thus the of correct by polypeptides in a environment that is in folding of or defective secretory proteins to they with cell and and maintain of the endomembrane system by of defective A of ER quality control in will to a of the in this In defective secretory proteins have been shown to be in the et al., in the vacuole et al., 1996; and and in an compartment of the secretory pathway that may be the ER itself and 1998). In cells, by quality control has been shown in the et al., and in lysosomes et al., through the pathway, and from the ER into the both and et al., a of the translocon pore, as well as et al., 1997). It may thus be that the translocon for translocation into the ER also defective proteins into the for In of proteins to the vacuole is via the Golgi complex, and is on a that also in the of the enzyme et al., 1996; and 1998). However, that proteins and are recognized by two of the lumenal et al., in the ER a is and and 1998). in may on in the et al., 1997). These different of are protein and they different of in the proteins, that for by or pathway be must defective proteins from the of polypeptides that are in the of that the but not the are The of the and a defective protein has been directly to the of the defective that the of unfolded from chaperone may for et al., 1998). a in to assemble as in with et al., 1997). Because is the of to in the be in the however, is as as the for of the to the The protein further to be in the ER, not of its and is in a that is to A and et al., both of to the of with a from of plants or the et al., labeled for with The with and the proteins by and The associated with which is therefore of its for the of an ER quality control system in from with a of the protein that to its in et a with an into but is in the of plants et al., the this the Thus, ER quality control not its through the Golgi complex, and for It to be the the ER or the vacuoles, by a pathway. The is to through the Golgi complex, the that its in vacuoles. In this an is its to is to by quality control or to the that et al., ER quality control also be in the of polypeptides in the of and start to be synthesized and in protein before the and The the protein which the (see and 1999, in this issue). polypeptides are in and to be more are and et al., 1996; et al., 1997). that and and et al., 1996; et al., 1997). protein in the of plants are mainly to the ER, but they are also within by to be a of The protein are in are the protein of the ER, of et al., It is not known the are in vacuoles, but the that they are by with that ER quality control be in the of that to their is that the of or defective proteins within the ER from their for as molecular and Because but not (Vitale et al., for is that this to the different of the and of this In the different to at during protein along and It has also been to their may a in the ER that an for newly synthesized proteins and Helenius, is as of a plant protein from the ER to the for However, the of into the by ER quality control to be by the a protein in the of when its and 1998). is of two by a The A is the the is a that to the surface of is by endocytosis and retrograde trafficking via the secretory pathway, as as the ER, from which the its into the to and 1998; for of the retrograde pathway, see Battey et al., 1999; Sanderfoot and Raikhel, 1999, in this issue). The is but the is that a molecules the be to the cell. Studies on the that the cell from that can also occur in plant et al., In is into the ER as a single inactive which is into the A and when the protein the via the Golgi the A is with its signal peptide in cells, is introduced into the ER and then in a to et al., this has on the in which is of the A and both with a signal peptide, in of the in the ER, along the secretory pathway, and in These that the A can from the ER into the also in plant and that the is by ER quality the that the plant vacuole is a point from which retrograde into the ER occur et al., Protein and ER In this only and are The two proteins with a newly synthesized protein is and involves of and and to the protein within the ER. A defective protein with the ER residents is eventually for by quality control mechanisms see The of the are also The first in protein into the secretory pathway is the of from the ER for to the Golgi This is with to in the lumen of the ER that to the of protein into the of protein the of within the of secretory in the of protein for secretory proteins to in the of the model is in an before from the ER. A is that the of the of model for protein has more recently in of that within the secretory pathway can occur and et al., In Munro and that of the of to of the and the of in was be secreted by they that to the the ER, or the Golgi For the or a of is at the of soluble and is to ER when to proteins, its depends on the protein et al., et al., 1992; et al., This model that proteins into that and of the of of secretory proteins to soluble ER residents only proteins ER as well as are removed from the pathway via retrograde Removal be after of to the Golgi complex, and that ER via from the Golgi was The model is by the that proteins are secreted when they are into the ER lumen via fusion to signal et al., and the of is in to that of secretory proteins and These observations are proteins of et al., In of a signal from a protein to of the et al., et al., of the pathway via also to et al., 1994; et al., that is for proteins to reach the cell not the two in in the ER or the Golgi of the the for a and was from soluble proteins to membrane proteins for the ER of The model a that to proteins and secreted to the of a vesicle proteins do not to the but can be in the lumen of the The model vesicle by a membrane-spanning protein that with the ER residents as well as secreted and proteins are in the vesicle by The first that may not be for of all proteins only to a of the model for ER The is based on the of vesicle budding in from ER be directly from ER to of proteins as et al., This was an discovery has been that ER is to from the Golgi The that of secretory during the of on the ER membrane, which the The that of the ER signal of only to and Pelham, can now be is not to an ER The of the to not for aqueous lumen the lumenal side with proteins an ER signal to the and be in the proteins have only a lumen into which to The of proteins in the may therefore be that is the the to in et al., In of the the ER signal of thus as a system for the of molecules that of the ER in A in of the model is from the that secretory proteins in the Golgi et al., However, of from the Golgi to the ER or on to the only the secretory proteins and also in an or of the The model the of an ER which has to be However, on the side of the ER membrane that to protein and the of are well (see Sanderfoot and Raikhel, 1999, in this issue). is an protein in this with a protein that the of the to the membrane. is a membrane-spanning protein with a lumenal but is not in et al., In the plant of the lumenal of the protein et al., an then be to with which thus to a to a via or In to a the model the of an ER signal on secretory and a have to be from the of the not ER et al., et al., et al., A signal for the of membrane proteins from the ER has been identified on the of and 1997). for the of soluble proteins, on the other have to be recognized on the lumenal side It is to the of a signal of a secretory protein that in its in the ER or of secretory can also be ER The that with et al., the of of its ER signal in et al., 1998; and and the of in and Pelham, in cells, have ER and and Helenius, that the of a protein complex, associated with the translocon Because the of may well exist in et al., not be to into nascent as the of the protein be to further on into is before can be to which of the two is the of more pathway for of membrane proteins from the ER have been and Jiang and Rogers, 1998). pathway is to the is observations on the of soluble proteins also the that are different for proteins to the ER. In of or to of proteins the ER et al., 1998). also within vesicles, at a with to the core of the of proteins to the is not by an of The therefore a model in which proteins are to the vesicles, bypassing the Golgi to this travel from the ER to the Golgi before at the of the to of the into the vacuoles, by fusion or by then The of from the at the of budding from the ER to be but this model two different of the ER, only of which is destined to the Golgi complex, and a that the proteins from other proteins in the ER. proteins structures in the ER that are to be through the Golgi and in et al., The that and proteins may different in the secretory pathway was also after observations of and is by the of a in the intermediate but also in this this to be and The of different of the ER that both for from the ER are by et be for proteins with ER occur via of vesicle that to be The recycling for of proteins as was first by fusion of the ER signal to the of the enzyme The protein in the ER but of passage through the This the of a that to ER in the Golgi to retrograde to the ER. The discovery of a for in and subsequent with the and Pelham, the The of in that from the ER is and that the system to proteins that have by This is also by the that in different plant the of more that of the itself et al., for a retrograde pathway in plants has only recently been with the of protein to the of et al., 1998). of plant with the fusion protein was into the ER, a that not have via in the ER. the recycling which is now A recycling model was also for membrane-spanning proteins a proteins have been shown to from the Golgi to the ER in retrograde and 1994; see also Sanderfoot and Raikhel, 1999, in this issue). However, was also shown that not a for retrograde traffic et al., 1993). that is to the Golgi after the of proteins to the ER. of must be and is the of ER of the ER system is only in and plants et al., but will be to the of during The two of ER have for the ER quality control and ER The of proteins be by the to of the ER of of proteins into vesicles, of the model However, the model a more of and that proteins as a role in the of protein into their not and they from the Golgi via the The ER signal of be recognized in the Golgi complex, and the protein be to the ER by with a of a plasma membrane protein is into the ER from the complex, in with and Helenius, are to this is in compartments. for The ER lumen ER residents and for and is in of ER residents from the Golgi to of the as by the in in at the vesicle budding from the ER, to the two soluble plant proteins have the but their in compartments other the the et al., and the et al., 1989). is located in the however, or of is on the cell surface et al., the protein as an It has been that this of at the cell surface be a of the of the However, the from occur through with be et al., 1997). is a protein that is in the of proteins during which its also when at the cell is of the et al., Removal of the is the first event in the of et al., This intermediate is and is located in an as compartment from the ER, that in the ER and allows from or after the has the ER. of involves of an N-terminal and of the A of more to the that the signal to from the ER et al., may thus be synthesized as a inactive The may thus also in the of both and and in other proteins, to maintain a of inactive protein that can be to its of only when It has also been that of ER be a more to of proteins via their to compartments et al., The plant ER the through the proteins The of plant will this compartment to do as proteins for and The that the plant ER is the of this is the of secretory A which is in and In is secreted by plasma as a of molecules in which of two and two is to its by a The is recognized by a on the surface of and secreted through at the surface into is by the of its to a secretory that associated with The secretory is thought to from in the plants and the and the secretory have been by with and of the plants et al., the assemble that to of the a that from a system in Thus, the assembly of which in at two different (the ER of plasma and the extracellular environment of is in the ER of an plant cell. In this is that the ER has that are intermediate the and the extracellular the of has been in et al., 1998). the years, on the plant secretory pathway has a with in the of the ER in protein and its for molecular has been of the this the during this have in cell and have plants as model for those for further is the of quality control by the ER, and along this will to to protein that will be of in both and is known the of proteins in the of the ER in and the of functional also a for the The of as which to the ER in has to be a et al., and will to be in the will from further different model and in this on the plant ER will an role in to to only be by to and in other the past and of for their and their many also and for are to and Robinson for during the is to for and Pedrazzini for the in the has been by from the on the and and the