The heat shock protein 70 molecular chaperone network in the pancreatic endoplasmic reticulum - a quantitative approach

Traditionally, the canine pancreatic endoplasmic reticulum (ER) has been the workhorse for cell-free studies on protein transport into the mammalian ER. These studies have revealed multiple roles for the major ER-luminal heat shock protein (Hsp) 70, IgG heavy chain-binding protein (BiP), at least one of which also involves the second ER-luminal Hsp70, glucose-regulated protein (Grp) 170. In addition, at least one of these BiP activities depends on Hsp40. Up to now, five Hsp40s and two nucleotide exchange factors, Sil1 and Grp170, have been identified in the ER of different mammalian cell types. Here we quantified the various proteins of this chaperone network in canine pancreatic rough microsomes. We also characterized the various purified proteins with respect to their affinities for BiP and their effect on the ATPase activity of BiP. The results identify Grp170 as the major nucleotide exchange factor for BiP, and the resident ER-membrane proteins ER-resident J-domain protein 1 plus ER-resident J-domain protein 2/Sec63 as prime candidates for cochaperones of BiP in protein transport in the pancreatic ER. Thus, these data represent a comprehensive analysis of the BiP chaperone network that was recently linked to two human inherited diseases, polycystic liver disease and Marinesco-Sjogren syndrome.