SIGNAL-MEDIATED SORTING OF MEMBRANE-PROTEINS BETWEEN THE ENDOPLASMIC-RETICULUM AND THE GOLGI-APPARATUS

Each organelle of the secretory pathway is required to selectively all ow transit of newly synthesized secretory and plasma membrane proteins and also to maintain a unique set of resident proteins that define it s structural and functional properties. In the case of the endoplasmic reticulum (ER), residency is achieved in two ways: (a) prevention of residents from entering newly forming transport vesicles and (b) retri eval of those residents that escape. The latter mechanism is directed by discrete retrieval motifs: Soluble proteins have a H/KDEL sequence at their carboxy-terminus; membrane proteins have a dibasic motif, eit her di-lysine or di-arginine, located close to the terminus of their c ytoplasmic domain. Recently it was found that di-lysine motifs bind th e complex of cytosolic coat proteins, COP I, and that this interaction functions in the retrieval of proteins from the Golgi to the ER. Also discussed are the potential roles this interaction may have in vesicu lar trafficking.