The role of molecular chaperones in protein transport into the mammalian endoplasmic reticulum.

Molecular chaperones belonging to the Hsp70 protein family play various roles in posttranslational protein transport into the yeast endoplasmic reticulum (ER): (i) Cytosolic Hsp70 (Hsc70) together with the J-domain containing Hsp40 preserves the transport competent state of presecretory proteins; (ii) ER-lumenal Hsp70 (Kar2p or Lhs1p) triggers the initial insertion of precursor proteins into the core of the protein translocase in the ER-membrane and cooperates with a J-domain containing membrane protein (Sec63p); (iii) the ER-lumenal Hsp70 and its membrane receptor also are involved in completion of translocation. Here a working model is presented for the putative roles of molecular chaperones in post- as well as cotranslational protein transport into the mammalian ER.