COPI-independent Golgi-to-ER retrograde traffic

In addition to the better characterized COPI-dependent retrograde Golgi-to-ER pathway, a second COPI-independent pathway has also been identified. This pathway is RAB6 dependent and transports cargo such as glycosylation enzymes and Shiga and Shiga-like toxin through tubular carriers rather than vesicles (White et al, 1999; Girod et al, 1999; reviewed in Heffernan and Simpson, 2014). In the absence of a COPI coat, the membrane curvature necessary to initiate tubulation may be provided through the action of phospholipase A, which hydrolyzes phospholipids at the sn2 position to yield lysophospholipids. This activity is countered by lysophospholipid acyltransferases, and the balance of these may influence whether transport tubules or transport vesicles form (de Figuiredo et al, 1998; reviewed in Bechler et al, 2012). RAB6-dependent tubules also depend on the dynein-dynactin motor complex and the hoomodimeric Bicaudal proteins (Matanis et al, 2002; Yamada et al, 2013; reviewed in Heffernan and Simpson, 2014).

除已明确定义的依赖COPI的逆行高尔基体至内质网通路之外，尚有一独立于COPI的通路亦被识别。该通路依赖于RAB6，并借助管状载体而非囊泡运输诸如糖基化酶和志贺毒素及其类似物等货物（参见White等人，1999年；Girod等人，1999年；Heffernan与Simpson，2014年综述）。在缺乏COPI包膜的情况下，启动管状结构所需的膜曲率可能由磷脂酶A的作用提供，该酶在sn2位置水解磷脂以生成溶血磷脂。此活性被溶血磷脂酰基转移酶所抵消，而两者之间的平衡可能影响运输管状结构或运输囊泡的形成（参见de Figuiredo等人，1998年；Bechler等人，2012年综述）。RAB6依赖的管状结构亦依赖于动粒-动力蛋白复合体和二聚体Bicaudal蛋白（参见Matanis等人，2002年；Yamada等人，2013年；Heffernan与Simpson，2014年综述）。