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Previous studies have shown that the rough endoplasmic reticulum (ER) contains nascent precursor polypeptide gated channels. Circumstantial evidence suggests that these channels are formed by the Sec61p complex. We reconstituted the purified Sec61p complex in a lipid bilayer and characterized its dynamics and regulation. The Sec61p complex is sufficient to form the precursor polypeptide activated channel under co- and posttranslational transport conditions. Activity of the Sec61p channel in both transport modes is induced by direct interaction with precursor protein. The Sec61p complex comprises a highly dynamic pore covering conductances corresponding to channel openings from approximately 6 to 60 A. Its properties are indistinguishable from those we observed with native ER channels, directly demonstrating that these channels are formed by the Sec61p complex.

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.