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

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Abstract
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.