Signals for Retention of Cytochrome P450 in the Endoplasmic Reticulum

According to the current model for the subcellular compartmentalization of proteins, any protein entering the secretory pathway will be transported by bulk flow to the cell surface unless a signal mediating its organelle-specific retention is present. Endoplasmic reticulum (ER) membrane protems, such as cytochromes P450, must have a sequence or structural features that prevent transport from the ER. For some ER membrane proteins, such signals have been identified and are encoded by a short amino acid sequence such as a double lysme motif at the C-terminus of the cytoplasmic tail or positively charged residues at the N-terminus (reviewed in ref. 1 ). ER retention signals may mediate either the direct retention (exclusion from further transport) or retrieval from a post-ER compartment (1 ).