Department of Medical Genetics, Centre for Molecular Medicine and Therapeutics, BC Children's Hospital Research Institute, University of British Columbia, Vancouver, Canada.
Traffic. 2018 Apr;19(4):285-295. doi: 10.1111/tra.12553. Epub 2018 Mar 4.
The polytopic yeast protein Chs3 (chitin synthase III) relies on a dedicated membrane-localized chaperone, Chs7, for its folding and expression at the cell surface. In the absence of Chs7, Chs3 forms high molecular weight aggregates and is retained in the endoplasmic reticulum (ER). Chs7 was reported to be an ER resident protein, but its role in Chs3 folding and transport was not well characterized. Here, we show that Chs7 itself exits the ER and localizes with Chs3 at the bud neck and intracellular compartments. We identified mutations in the Chs7 C-terminal cytosolic domain that do not affect its chaperone function, but cause it to dissociate from Chs3 at a post-ER transport step. Mutations that prevent the continued association of Chs7 with Chs3 do not block delivery of Chs3 to the cell surface, but dramatically reduce its catalytic activity. This suggests that Chs7 engages in functionally distinct interactions with Chs3 to first promote its folding and ER exit, and subsequently to regulate its activity at the plasma membrane.
多叶酵母蛋白 Chs3（几丁质合成酶 III）依赖于一种专门的膜定位伴侣 Chs7 来进行折叠和在细胞表面表达。在没有 Chs7 的情况下，Chs3 会形成高分子量聚集体并滞留在内质网（ER）中。Chs7 被报道为内质网驻留蛋白，但它在 Chs3 折叠和运输中的作用尚未得到很好的表征。在这里，我们表明 Chs7 本身会离开内质网，并与 Chs3 一起定位于芽颈和细胞内隔室。我们鉴定了 Chs7 C 端胞质结构域中的突变，这些突变不影响其伴侣功能，但会导致其在 ER 后运输步骤中与 Chs3 分离。阻止 Chs7 与 Chs3 持续关联的突变不会阻止 Chs3 递送到细胞表面，但会极大地降低其催化活性。这表明 Chs7 与 Chs3 进行功能上不同的相互作用，首先促进其折叠和 ER 输出，随后调节其在质膜上的活性。
