Chen M, Zhang J T
Department of Pharmacology and Toxicology, IU Cancer Center, Indiana University School of Medicine, Indianapolis 46202, USA.
Biochemistry. 1999 Apr 27;38(17):5471-7. doi: 10.1021/bi982153t.
Cystic fibrosis transmembrane conductance regulator (CFTR) is a member of the ATP-binding cassette (ABC) transport superfamily. CFTR folding and assembly appear to involve several events occurred in the cytosol and ER. Misfolding of CFTR causes cystic fibrosis, and thus, understanding the folding mechanism of CFTR is extremely important. Recently, detailed study of membrane insertion process suggests that the first two transmembrane (TM) segments of CFTR have two distinct but independent mechanisms to ensure the correct membrane folding of its amino terminal end [Lu, Y., Xiong, X., Helm, A., Kimani, K., Bragin, A., Skach, W. R. (1998) J. Biol. Chem. 273, 568-576]. To understand how other TM segments are ensured to insert into membranes correctly, we investigated the topogenesis of TM3 and TM4 of CFTR in a cell-free expression system. We found that the correct membrane insertion of TM3 and TM4 of CFTR was ensured by their flanking amino acid sequences and controlled by the correct membrane insertion of their preceding TM1 and TM2. Thus, correct membrane insertion and folding of TM1 and TM2 play an essential role in the membrane insertion and folding of the subsequent TM segments of CFTR.
囊性纤维化跨膜传导调节因子（CFTR）是ATP结合盒（ABC）转运超家族的成员。CFTR的折叠和组装似乎涉及在细胞质和内质网中发生的几个事件。CFTR的错误折叠会导致囊性纤维化，因此，了解CFTR的折叠机制极其重要。最近，对膜插入过程的详细研究表明，CFTR的前两个跨膜（TM）片段有两种不同但独立的机制来确保其氨基末端的正确膜折叠[Lu, Y., Xiong, X., Helm, A., Kimani, K., Bragin, A., Skach, W. R. (1998) J. Biol. Chem. 273, 568 - 576]。为了了解其他TM片段如何确保正确插入膜中，我们在无细胞表达系统中研究了CFTR的TM3和TM4的拓扑形成。我们发现，CFTR的TM3和TM4的正确膜插入是由其侧翼氨基酸序列确保的，并受其前面的TM1和TM2的正确膜插入控制。因此，TM1和TM2的正确膜插入和折叠在CFTR后续TM片段的膜插入和折叠中起着至关重要的作用。
