Hsp70: J-domain protein (JDP) machines, along with the cellular protein degradation systems play a central role in regulating cellular proteostasis. An equally robust surveillance system operates at the plasma membrane too that affects proper sorting, stability as well as the turnover of membrane proteins. Although plausible, a definitive role of the Hsp70: JDP machine in regulating the stability of plasma membrane proteins is not well understood in Saccharomyces cerevisiae. Here we show that a moderate over-expression of Caj1, one of the thirteen JDPs residing in the nucleo-cytosolic compartment of S. cerevisiae reduced the cold sensitivity of tryptophan auxotrophic yeast cells by stabilizing tryptophan permeases, Tat1 and Tat2 in a J-domain dependent manner. Concomitantly, higher Caj1 levels also caused slow growth and increased plasma membrane damage at elevated temperatures possibly due to the stabilization of thermolabile plasma membrane proteins. Finally, we show that although majorly cytosolic, Caj1 also co-localizes with the membrane dye FM4-64 at the cellular periphery suggesting that Caj1 might interact with the plasma membrane. Based on the results presented in this study, we implicate the Hsp70: Caj1 chaperone machine in regulating the stability or turnover of plasma membrane proteins in budding yeast.

Hsp70：J结构域蛋白（JDP）机器以及细胞蛋白降解系统在调节细胞蛋白稳态中起着核心作用。同样强大的监视系统也会在质膜上运行，这会影响适当的分类，稳定性以及膜蛋白的周转。尽管是合理的，但在酿酒酵母中尚不清楚Hsp70：JDP机器在调节质膜蛋白稳定性方面的确定作用。在这里，我们显示了Caj1的适度过表达，Caj1是S的胞质区中的13个JDP之一。酿酒酵母通过以J结构域依赖性方式稳定色氨酸通透酶Tat1和Tat2来降低色氨酸营养缺陷型酵母细胞的冷敏感性。随之而来的是，较高的Caj1水平也会在高温下引起缓慢的生长并增加质膜的破坏，这可能是由于对热不稳定的质膜蛋白的稳定作用所致。最后，我们显示，尽管Caj1主要是胞质的，但它也与膜染料FM4-64在细胞外围共定位，提示Caj1可能与质膜相互作用。基于这项研究中提出的结果，我们暗示Hsp70：Caj1伴侣机在调节发芽酵母中质膜蛋白的稳定性或周转率。