RETRIEVAL OF TRANSMEMBRANE PROTEINS TO THE ENDOPLASMIC-RETICULUM

A COOH-terminal double lysine motif maintains type I transmembrane pro teins in the ER. Proteins tagged with this motif, eg., CD8/E19 and CD4 /E19, rapidly receive post-translational modifications characteristic of the intermediate compartment and partially colocalized to this orga nelle. These proteins also received modifications characteristic of th e Golgi but much more slowly. Lectin staining localized these Golgi mo dified proteins to ER indicating that this motif is a retrieval signal . Differences in the subcellular distribution and rate of post-transla tional modification of CD8 maintained in the ER by sequences derived f rom a variety of ER resident proteins suggested that the efficiency of retrieval was dependent on the sequence context of the double lysine motif and that retrieval may be initiated from multiple positions alon g the exocytotic pathway.