Calsequestrin isoforms localize to different ER subcompartments: evidence for polymer and heteropolymer-dependent localization.

Skeletal muscle calsequestrin (skelCSQ) and cardiac calsequestrin (cardCSQ) are resident proteins of the ER/SR, but mechanisms by which CSQ is retained inside membrane lumens remain speculative. A structural model that predicts linear CSQ polymers has been developed that might explain CSQ concentration and localization inside junctional SR lumens, however little evidence exists for polymer formation in intact cells or for its effects on subcellular localization. We previously showed that cardCSQ is efficiently retained within the ER, but its retention is lost under conditions expected to disrupt its polymerization. In the present study, we found unexpectedly that skelCSQ shows no co-localization with cardCSQ in COS cells or in rat neonatal heart cells, but instead concentrates in a membrane compartment (ERGIC) that is just distal to that of cardCSQ. Consistent with this difference in immunofluorescent localization, the structures of CSQ ((316)Asn-linked) glycans showed two types of pre-Golgi processing. Despite the difference in subcellular distribution of individual wild-type forms of CSQ, however, pairs of different CSQ molecules (for example, different isoforms or different fluorescent fusion proteins) consistently co-localized, suggesting that separate forms of CSQ polymerize in different parts of the same secretory pathway, while different CSQ pairs localize together through heteropolymerization.