Wang C, Chen M, Han E, Zhang J T
Department of Physiology and Biophysics, University of Texas Medical Branch, Galveston, Texas 77555-0641, USA.
Biochemistry. 1997 Sep 23;36(38):11437-43. doi: 10.1021/bi970041g.
The topogenesis of membrane proteins with a single transmembrane (TM) segment is well understood. However, understanding the topogenesis and membrane assembly of membrane proteins with multiple TM segments (polytopic) is still incomplete. Recently, several studies on P-glycoprotein (Pgp) suggested that the topogenesis of polytopic membrane proteins is likely more complicated than anticipated. While studying the mechanism by which Pgp topogenesis is determined, we unexpectedly found that ribosomes or proteins associated with ribosomes are involved in regulating the membrane insertion and folding of Pgp during its translation. We discovered that when Pgp was translated by wheat germ ribosomes in vitro, TM3 could not reinitiate the insertion of the protein into microsomal membranes following the membrane insertion of TM1 and TM2. In contrast, TM3 could reinitiate membrane insertion when the protein was translated by rabbit reticulocyte ribosomes. These findings suggest that ribosomes or proteins associated with ribosomes play an important role in membrane insertion and folding of TM segments of Pgp and that rabbit reticulocyte and wheat germ ribosomes may use different mechanisms to control the membrane insertion of the same nascent peptide. We propose that ribosomes or proteins associated with ribosomes help reinitiate insertion of internal TM segments into the membrane by dissociation and reassociation with the protein-conducting channel in ER membranes.
具有单个跨膜（TM）区段的膜蛋白的拓扑形成已得到充分理解。然而，对于具有多个TM区段（多跨膜）的膜蛋白的拓扑形成和膜组装的理解仍不完整。最近，几项关于P-糖蛋白（Pgp）的研究表明，多跨膜膜蛋白的拓扑形成可能比预期的更复杂。在研究Pgp拓扑形成的决定机制时，我们意外地发现核糖体或与核糖体相关的蛋白质在Pgp翻译过程中参与调节其膜插入和折叠。我们发现，当Pgp在体外由小麦胚芽核糖体翻译时，在TM1和TM2插入膜后，TM3无法重新启动蛋白质插入微粒体膜的过程。相反，当蛋白质由兔网织红细胞核糖体翻译时，TM3可以重新启动膜插入。这些发现表明，核糖体或与核糖体相关的蛋白质在Pgp的TM区段的膜插入和折叠中起重要作用，并且兔网织红细胞和小麦胚芽核糖体可能使用不同的机制来控制同一新生肽的膜插入。我们提出，核糖体或与核糖体相关的蛋白质通过与内质网膜中的蛋白质传导通道解离和重新结合，帮助内部TM区段重新插入膜中。
