Mutations in the yeast Hsp40 chaperone protein Ydj1 cause defects in Axl1 biogenesis and pro-a-factor processing

The heat shock protein (Hsp) 70/Hsp40 chaperone system plays an essential r ole in cell physiology, but few of its in vivo functions are known, We repo rt that biogenesis of Axl1p, an insulinase-like endoprotease from yeast, is dependent upon the cytosolic Hsp40 protein Ydj1p, Axil is responsible for cleavage of the P2 processing intermediate of pro-a-factor, a mating pherom one, to its mature form. Mutant ydj1 strains exhibited a severe mating defe ct, which correlated with a 90% reduction in a-factor secretion. Reduced le vels of a-factor export were caused by defects in the endoproteolytic proce ssing of P2, which led to its intracellular accumulation. Defective P2 proc essing correlated with the reduction in the steady state level of active Ax l1p, Two mechanisms were uncovered to explain why Axl1p activity was dimini shed in ydj1 strains. First, AXL1 mRNA levels were reduced ydj1 strains. Se cond, the half-life of newly synthesized Axl1p was greatly diminished in yd j1 strains. Collectively, these data indicate Ydj1p functions to promote AX L1 mRNA accumulation and in addition appears to facilitate the proper foldi ng of nascent Axl1p, This study is the first to suggest a role Sor Pdj1p in RNA metabolism and identifies Axl1p as an in vivo substrate of the Hsp70/Y dj1p chaperone system.