School of Physiology and Pharmacology, Bristol Heart Institute and Centre for Nanoscience and Quantum Information, University of Bristol, Bristol, United Kingdom.
Biochemistry. 2011 Apr 5;50(13):2623-32. doi: 10.1021/bi1019447. Epub 2011 Mar 7.
Mitsugumin 23 (MG23) is a 23 kDa transmembrane protein localized to the sarcoplasmic/endoplasmic reticulum and nuclear membranes in a wide variety of cells. Although the characteristics imply the participation in a fundamental function in intracellular membrane systems, the physiological role of MG23 is unknown. Here we report the biochemical and biophysical characterization of MG23. Hydropathicity profile and limited proteolytic analysis proposed three transmembrane segments in the MG23 primary structure. Chemical cross-linking analysis suggested a homo-oligomeric assembly of MG23. Ultrastructural observations detected a large symmetrical particle as the predominant component and a small asymmetric assembly as the second major component in highly purified MG23 preparations. Single-particle three-dimensional reconstruction revealed that MG23 forms a large bowl-shaped complex equipped with a putative central pore, which is considered an assembly of the small asymmetric subunit. After reconstitution into planar phospholipid bilayers, purified MG23 behaved as a voltage-dependent, cation-conducting channel, permeable to both K(+) and Ca(2+). A feature of MG23 gating was that multiple channels always appeared to be gating together in the bilayer. Our observations suggest that the bowl-shaped MG23 can transiently assemble and disassemble. These building transitions may underlie the unusual channel gating behavior of MG23 and allow rapid cationic flux across intracellular membrane systems.
Mitsugumin 23（MG23）是一种 23 kDa 的跨膜蛋白，定位于多种细胞的肌浆内质网和核膜。尽管其特征表明其参与了细胞内膜系统的基本功能，但 MG23 的生理作用尚不清楚。本文报道了 MG23 的生化和生物物理特性。疏水性分析和有限的蛋白水解分析表明，MG23 的一级结构中有三个跨膜区。化学交联分析表明 MG23 是同源寡聚体组装。超微结构观察在高度纯化的 MG23 制剂中检测到一种大的对称颗粒作为主要成分，以及一种小的不对称组装作为第二种主要成分。单颗粒三维重构表明，MG23 形成一个带有中央孔的大型碗状复合物，被认为是小的不对称亚基的组装。在重新组装到平面磷脂双层后，纯化的 MG23 表现为一种电压依赖性、阳离子导电通道，对 K(+)和 Ca(2+)都具有通透性。MG23 门控的一个特征是，在双层中，多个通道似乎总是同时门控。我们的观察表明，碗状的 MG23 可以暂时组装和拆卸。这些结构转换可能是 MG23 异常通道门控行为的基础，并允许阳离子在细胞内膜系统中快速流动。
