A novel type of co-chaperone mediates transmembrane recruitment of DnaK-like chaperones to ribosomes.

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 reticulumcontains 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 domainand a cytosolic domain which is in close contact with translating ribosomesand appears to have the ability to modulate translation. The interaction with ribosomesinvolves 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 ribosomesand, possibly, transmembrane signaling between ribosomesand DnaK-like chaperones of the endoplasmic reticulum.