Cardiac ER/SR lumens contain numerous soluble proteins that function predominantly in protein homeostasis. Many of these luminal proteins exhibit long stretches of acidic amino acids (Glu or Asp) with embedded serine residues that are efficient substrates for protein kinase CK2. The role of calsequestrin-2 (CSQ2) as a major Ca2+-binding protein component of excitation-contraction coupling has been challenged by the lack of effects in the CSQ2 null mouse, yet no other models of CSQ2 function have been proposed. CSQ2 is phosphorylated on virtually every molecule, as verified by intact protein electrospray mass spectrometry. Analysis of CSQ2 from heart tissue samples show that molecules with the highest levels of phosphate are those most recently translocated from the cytosol to the ER lumen, as if phosphorylation occurred in the cytosol just before translocation. A new paradigm of CSQ2 actions is proposed in which newly synthesized CSQ2 molecules exit their ribosome tunnel and bind immediately to protein kinase CK2 in the cytosol. We hypothesize that the acidic tail of nascent CSQ2 acts as a unique and efficient regulator of CK2, inhibiting its activity under basal conditions. Under these conditions, CSQ2 fails to undergo ER translocation, likely due to the effect of CK2 binding on a weak CSQ2 signal peptide-sec61 interaction. We hypothesize that the tight binding of CSQ2 is reversed only when it becomes phosphorylated by CK2, leading to its dissociation. The increase in active CK2 in the cytosol stimulates hypertrophic growth. The soluble CSQ2 facilitates formation of the looped configuration needed for ER translocation. This model predicts that, under hypertrophic signals, CSQ2 quickly populates the ER/SR with newly folded proteins and newly processed glycans, a structural change in CSQ2 observed in multiple canine heart failure models.
Steven E. Cala (Sun,) studied this question.