Braunagel Sharon C, Williamson Shawn T, Ding Qi, Wu Xiaogiang, Summers Max D
Department of Biology, Texas Agricultural Experiment Station, Texas A&M University, College Station, TX 77843, USA.
Proc Natl Acad Sci U S A. 2007 May 29;104(22):9307-12. doi: 10.1073/pnas.0703186104. Epub 2007 May 21.
Spodoptera frugiperda (Sf9) importin-alpha-16 is a translocon-associated protein that participates in the early sorting pathway of baculovirus integral membrane proteins destined for the inner nuclear membrane (INM). To discern whether sorting intermediate protein complexes like those observed in insect cells are also formed with mammalian INM proteins, cross-linked complexes of importin-alpha-16 with human lamin B receptor (LBR) and nurim were examined. Both LBR and nurim cross-link with Sf9 importin-alpha-16 during cotranslational membrane integration and remain proximal with importin-alpha-16 after integration into the endoplasmic reticulum membrane and release from the translocon. Human cells encode several isoforms of importin-alpha; to determine whether any of these isoforms may recognize INM-directed proteins, they were tested for their ability to cross-link with the viral-derived INM sorting motif sequence. One cross-linked adduct was detected with a 16-kDa isoform encoded from KPNA4 (KPNA-4-16). KPNA-4-16 was easily detected in microsomal membranes prepared from KPNA4-16 recombinant virus-infected cells and was also detected in microsomes prepared from HeLa cells. Together these observations suggest that elements of the early sorting pathway of INM-directed proteins mediated by importin-alpha-16 are highly conserved, and mammalian KPNA-4-16 is a candidate partner in sorting integral membrane proteins to the INM.
草地贪夜蛾（Sf9）输入蛋白α-16是一种与转运体相关的蛋白质，它参与杆状病毒整合膜蛋白向内核膜（INM）的早期分选途径。为了确定在昆虫细胞中观察到的那种分选中间蛋白复合物是否也会与哺乳动物的INM蛋白形成，研究人员检测了输入蛋白α-16与人核纤层蛋白B受体（LBR）和核仁素的交联复合物。在共翻译膜整合过程中，LBR和核仁素均与Sf9输入蛋白α-16发生交联，并且在内质网膜整合并从转运体释放后，它们仍与输入蛋白α-16保持近端关系。人类细胞编码多种输入蛋白α的亚型；为了确定这些亚型中是否有任何一种可能识别靶向INM的蛋白质，研究人员测试了它们与病毒衍生的INM分选基序序列发生交联的能力。用KPNA4编码的一种16 kDa亚型（KPNA-4-16）检测到一种交联加合物。在由KPNA4-16重组病毒感染的细胞制备的微粒体膜中很容易检测到KPNA-4-16，在由HeLa细胞制备的微粒体中也检测到了它。这些观察结果共同表明，由输入蛋白α-16介导的靶向INM的蛋白质早期分选途径的元件高度保守，并且哺乳动物的KPNA-4-16是将整合膜蛋白分选到INM的候选伴侣。
