Meacham G C, Browne B L, Zhang W, Kellermayer R, Bedwell D M, Cyr D M
Department of Cell Biology, University of Alabama Medical Center, Birmingham, Alabama 35294-0005, USA.
J Biol Chem. 1999 Nov 26;274(48):34396-402. doi: 10.1074/jbc.274.48.34396.
The heat shock protein (Hsp) 70/Hsp40 chaperone system plays an essential role in cell physiology, but few of its in vivo functions are known. We report that biogenesis of Axl1p, an insulinase-like endoprotease from yeast, is dependent upon the cytosolic Hsp40 protein Ydj1p. Axl1 is responsible for cleavage of the P2 processing intermediate of pro-a-factor, a mating pheromone, to its mature form. Mutant ydj1 strains exhibited a severe mating defect, which correlated with a 90% reduction in a-factor secretion. Reduced levels of a-factor export were caused by defects in the endoproteolytic processing of P2, which led to its intracellular accumulation. Defective P2 processing correlated with the reduction in the steady state level of active Axl1p. Two mechanisms were uncovered to explain why Axl1p activity was diminished in ydj1 strains. First, AXL1 mRNA levels were reduced ydj1 strains. Second, the half-life of newly synthesized Axl1p was greatly diminished in ydj1 strains. Collectively, these data indicate Ydj1p functions to promote AXL1 mRNA accumulation and in addition appears to facilitate the proper folding of nascent Axl1p. This study is the first to suggest a role for Ydj1p in RNA metabolism and identifies Axl1p as an in vivo substrate of the Hsp70/Ydj1p chaperone system.
热休克蛋白（Hsp）70/Hsp40伴侣系统在细胞生理学中起着至关重要的作用，但其体内功能却鲜为人知。我们报道，酵母中一种胰岛素酶样内切蛋白酶Axl1p的生物合成依赖于胞质Hsp40蛋白Ydj1p。Axl1负责将交配信息素前体α-因子的P2加工中间体切割成成熟形式。Ydj1突变株表现出严重的交配缺陷，这与α-因子分泌减少90%相关。α-因子输出水平降低是由P2的内切蛋白水解加工缺陷导致的，这导致其在细胞内积累。有缺陷的P2加工与活性Axl1p稳态水平降低相关。我们发现了两种机制来解释为什么Ydj1株中Axl1p活性降低。首先，Ydj1株中AXL1 mRNA水平降低。其次，新合成的Axl1p在Ydj1株中的半衰期大大缩短。总的来说，这些数据表明Ydj1p的功能是促进AXL1 mRNA积累，此外似乎还促进新生Axl1p的正确折叠。这项研究首次表明Ydj1p在RNA代谢中的作用，并将Axl1p鉴定为Hsp70/Ydj1p伴侣系统的体内底物。
