ABSTRACT
Transport of presecretory proteins into the mammalian rough endoplasmic reticulum involves a protein translocase that comprises the integral membrane proteins Sec61alphap, Sec61betap, and Sec61gammap as core components. Electron microscopic analysis of protein translocase in rough microsomal membranes suggested that between three and four heterotrimeric Sec61p complexes form the central unit of protein translocase. Here we analyzed the stoichiometry of heterotrimeric Sec61p complexes present in cotranslationally active protein translocases of canine pancreatic microsomes and various other lumenal and membrane components believed to be subunits of protein translocase and to be involved in covalent modifications. Based on these numbers, the capacity for cotranslational transport was estimated for the endoplasmic reticulum of the human pancreas.
Subject(s)
Membrane Proteins/metabolism , Microsomes/metabolism , Pancreas/cytology , Pancreas/metabolism , Animals , Cell-Free System , Dogs , Endoplasmic Reticulum/metabolism , Humans , Protein Transport , SEC Translocation ChannelsABSTRACT
Recently, the homolog of yeast protein Sec63p was identified in dog pancreas microsomes. This pancreatic DnaJ-like protein was shown to be an abundant protein, interacting with both the Sec61p complex and lumenal DnaK-like proteins, such as BiP. The pancreatic endoplasmic reticulum contains a second DnaJ-like membrane protein, which had been termed Mtj1p in mouse. Mtj1p is present in pancreatic microsomes at a lower concentration than Sec63p but has a higher affinity for BiP. In addition to a lumenal J-domain, Mtj1p contains a single transmembrane domain and a cytosolic domain which is in close contact with translating ribosomes and appears to have the ability to modulate translation. The interaction with ribosomes involves a highly charged region within the cytosolic domain of Mtj1p. We propose that Mtj1p represents a novel type of co-chaperone, mediating transmembrane recruitment of DnaK-like chaperones to ribosomes and, possibly, transmembrane signaling between ribosomes and DnaK-like chaperones of the endoplasmic reticulum.
