Hsc70, immunoglobulin heavy chain binding protein, and Hsp90 differ in their ability to stimulate transport of precursor proteins into mammalian microsomes.

Ribonucleoparticle-independent transport of precursor proteins into mammalian microsomes is stimulated by 70-kDa heat shock proteins (Hsc70) and an additional cytosolic protein. Here we addressed the question of whether other molecular chaperones can replace Hsc70 in facilitating protein transport into the endoplasmic reticulum. Specifically, we asked if members of the same family of stress proteins, i.e. the microsomal protein immunoglobulin heavy chain binding protein or the bacterial protein DnaK, can substitute for Hsc70. Furthermore, we investigated whether molecular chaperones with a proven role in protein folding and belonging to the other two major families of stress proteins, i.e. Hsp60 or Hsp90, can substitute for Hsc70. We show that none of these stress proteins was able to substitute for Hsc70 in facilitating protein transport into mammalian microsomes. GroEL (the bacterial member of the Hsp60 family) and Hsp90, however, competed with Hsc70 for binding of the non-native precursor protein. Therefore, we conclude that there are both substrate and functional specificity in the action of molecular chaperones.