Calumenin and Reticulocalbin are Associated with the Protein Translocase of the Mammalian Endoplasmic Reticulum

Transport of precursor proteins into the Endoplasmic Reticulum (ER) requires signal peptides in the precursor proteins and a protein translocase in the ER membrane. Recent findings about the protein translocase of the mammalian ER suggest that the membrane is permeable to small molecules during protein translocation/insertion. Since the ER also plays a central role in calcium homeostasis the question arises of how these two activities of the ER are reconciled. Here two EF-hand proteins of the ER were characterized as associates of protein translocase. A model for the activity of these proteins in protein transport and calcium homeostasis is proposed.