van Eys G J, Völler M C, Timmer E D, Wehrens X H, Small J V, Schalken J A, Ramaekers F C, van der Loop F T
Department of Molecular Cell Biology and Genetics, Maastricht University, The Netherlands.
Cell Struct Funct. 1997 Feb;22(1):65-72. doi: 10.1247/csf.22.65.
Recently we described a protein, smoothelin, that has been exclusively found in smooth muscle cells (SMC). The human cDNA has been cloned from a colon cDNA library and the putative protein sequence was deduced. Smoothelin does not belong to a known protein family but shows a partial homology with members of the spectrin family. Transfection studies revealed that smoothelin has an affinity for actin and is either capable of forming filamentous structures or colocalizes with such structures. The protein is expressed in visceral as well as vascular tissues of all vertebrate classes. A study on the distribution of smoothelin in the vascular and placental system showed that smoothelin expression was largely restricted to the muscular pulsating blood vessels. Therefore, we hypothesized that smoothelin is expressed in contractile SMC only (36, 37). No expression of smoothelin was observed in established cell lines of SMC. In tissue explants smoothelin mRNA concentration decreases to undetectable levels within 12 hours after dissection as was in general the case in primary cell cultures. Here we report on continued smoothelin expression for several passages observed in a human prostate primary cell culture system. Smoothelin was demonstrated to colocalize with actin stress fibers but not with desmin filaments. This culture system offers opportunities to study the cytological localization of smoothelin, interactions with other proteins and should provide a system to test the promoter of the smoothelin gene. On immunoblots the molecular weight of smoothelin differed between visceral and vascular smooth muscle tissue with apparent molecular weights of respectively 59 kDa and 94 kDa. There is no evidence for the existence of another gene coding for the 94 kDa smoothelin. Thus, posttranslational modification, alternative splicing and dual promoter control are the alternatives for the expression of two isoforms of smoothelin.
最近，我们描述了一种仅在平滑肌细胞（SMC）中发现的蛋白质——平滑肌动蛋白。人cDNA已从结肠cDNA文库中克隆出来，并推导了推定的蛋白质序列。平滑肌动蛋白不属于已知的蛋白质家族，但与血影蛋白家族成员有部分同源性。转染研究表明，平滑肌动蛋白对肌动蛋白有亲和力，能够形成丝状结构或与这些结构共定位。该蛋白在所有脊椎动物类别的内脏和血管组织中均有表达。一项关于平滑肌动蛋白在血管和胎盘系统中分布的研究表明，平滑肌动蛋白的表达主要局限于肌肉搏动血管。因此，我们推测平滑肌动蛋白仅在收缩性SMC中表达（36, 37）。在已建立的SMC细胞系中未观察到平滑肌动蛋白的表达。在组织外植体中，平滑肌动蛋白mRNA浓度在解剖后12小时内降至无法检测的水平，原代细胞培养通常也是如此。在此，我们报告在人前列腺原代细胞培养系统中观察到平滑肌动蛋白在多个传代中持续表达。已证明平滑肌动蛋白与肌动蛋白应力纤维共定位，但与结蛋白丝不共定位。该培养系统为研究平滑肌动蛋白的细胞学定位、与其他蛋白质的相互作用提供了机会，并应提供一个测试平滑肌动蛋白基因启动子的系统。在免疫印迹上，平滑肌动蛋白的分子量在内脏和血管平滑肌组织之间有所不同，表观分子量分别为59 kDa和94 kDa。没有证据表明存在另一个编码94 kDa平滑肌动蛋白的基因。因此，翻译后修饰、可变剪接和双启动子控制是平滑肌动蛋白两种同工型表达的替代方式。
