Laboratory of Glycobiology, Fundación Instituto Leloir and Instituto de Investigaciones Bioquímicas de Buenos Aires, Buenos Aires, Argentina.
PLoS One. 2010 Oct 5;5(10):e13141. doi: 10.1371/journal.pone.0013141.
For most secretory pathway proteins, crossing the endoplasmic reticulum (ER) membrane is an irreversible process. However, in some cases this flow can be reversed. For instance, misfolded proteins retained in the ER are retrotranslocated to the cytosol to be degraded by the proteasome. This mechanism, known as ER associated degradation (ERAD), is exploited by several bacterial toxins to gain access to the cytosol. Interestingly, some ER resident proteins can also be detected in the cytosol or nucleus, calreticulin (CRT) being the most studied. Here we show that in Trypanosoma cruzi a minor fraction of CRT localized to the cytosol. ER calcium depletion, but not increasing cytosolic calcium, triggered the retrotranslocation of CRT in a relatively short period of time. Cytosolic CRT was subsequently degraded by the proteasome. Interestingly, the single disulfide bridge of CRT is reduced when the protein is located in the cytosol. The effect exerted by ER calcium was strictly dependent on the C-terminal domain (CRT-C), since a CRT lacking it was totally retained in the ER, whereas the localization of an unrelated protein fused to CRT-C mirrored that of endogenous CRT. This finding expands the regulatory mechanisms of protein sorting and may represent a new crossroad between diverse physiological processes.
对于大多数分泌途径的蛋白质来说，穿过内质网（ER）膜是一个不可逆的过程。然而，在某些情况下，这种流动可以被逆转。例如，滞留在 ER 中的错误折叠的蛋白质被反向转运到细胞质中，被蛋白酶体降解。这种机制被称为 ER 相关降解（ERAD），被几种细菌毒素利用以进入细胞质。有趣的是，一些 ER 驻留蛋白也可以在细胞质或核中被检测到，钙网织蛋白（CRT）是研究最多的一种。在这里，我们表明在克氏锥虫中，一小部分 CRT 定位于细胞质中。内质网钙耗竭，但细胞质钙增加不会触发 CRT 的反向转运，这一过程发生在相对较短的时间内。细胞质 CRT 随后被蛋白酶体降解。有趣的是，当蛋白质位于细胞质中时，CRT 的单个二硫键被还原。内质网钙所施加的影响严格依赖于 C 末端结构域（CRT-C），因为缺乏 CRT-C 的 CRT 完全保留在内质网中，而与 CRT-C 融合的无关蛋白的定位则反映了内源性 CRT 的定位。这一发现扩展了蛋白质分拣的调控机制，可能代表了不同生理过程之间的一个新的交汇点。
