MAMMALIAN VESICLE TRAFFICKING PROTEINS OF THE ENDOPLASMIC-RETICULUM AND GOLGI-APPARATUS

Vesicle traffic propagates and maintains distinct subcellular compartm ents and routes secretory products from their site of synthesis to the ir final destinations. As a basis for the specificity of vesicular tra nsport reactions, each step in the secretory pathway appears to be han dled by a distinct set of evolutionarily conserved proteins. Mammalian proteins responsible for vesicle trafficking at early steps in the se cretory pathway are not well understood. In this report, we describe r at sec22 (rsec22) and rat bet1 (rbet1), mammalian sequence homologs of yeast proteins identified as mediators of endoplasmic reticulum-to-Go lgi protein transport. rsec22 and rbet1 were expressed widely in mamma lian tissues, as anticipated for proteins involved in fundamental memb rane trafficking reactions. Recombinant rsec22 and rbet1 proteins beha ved as integral membrane components of 28 and 18 kDa, respectively, co nsistent with their primary structures, which contain a predicted tran smembrane domain at or near the carboxyl terminus. Recombinant rsec22 and rbet1 had distinct subcellular localizations, with rsec22 residing on endoplasmic reticulum membranes and rbet1 found on Golgi membranes . Studies with brefeldin A and nocodazole indicated that rbet1 functio n might involve interaction with or retention in the intermediate comp artment. The distinct localizations of rsec22 and rbet1 may reflect th eir participation in opposite directions of membrane flow between the endoplasmic reticulum and Golgi apparatus.