Audhya Anjon, McLeod Ian X, Yates John R, Oegema Karen
Ludwig Institute for Cancer Research, Department of Cellular and Molecular Medicine, University of California San Diego, La Jolla, California, USA.
PLoS One. 2007 Sep 26;2(9):e956. doi: 10.1371/journal.pone.0000956.
After ligand binding and endocytosis, cell surface receptors can continue to signal from endosomal compartments until sequestered from the cytoplasm. An important mechanism for receptor downregulation in vivo is via the inward budding of receptors into intralumenal vesicles to form specialized endosomes called multivesicular bodies (MVBs) that subsequently fuse with lysosomes, degrading their cargo. This process requires four heterooligomeric protein complexes collectively termed the ESCRT machinery. In yeast, ESCRT-I is a heterotetrameric complex comprised of three conserved subunits and a fourth subunit for which identifiable metazoan homologs were lacking. Using C. elegans, we identify MVB-12, a fourth metazoan ESCRT-I subunit. Depletion of MVB-12 slows the kinetics of receptor downregulation in vivo, but to a lesser extent than inhibition of other ESCRT-I subunits. Consistent with these findings, targeting of MVB-12 to membranes requires the other ESCRT-I subunits, but MVB-12 is not required to target the remaining ESCRT-I components. Both endogenous and recombinant ESCRT-I are stable complexes with a 1:1:1:1 subunit stoichiometry. MVB-12 has two human homologs that co-localize and co-immunoprecipitate with the ESCRT-I component TSG101. Thus, MVB-12 is a conserved core component of metazoan ESCRT-I that regulates its activity during MVB biogenesis.
配体结合和内吞作用后，细胞表面受体可在内体区室持续发出信号，直至与细胞质隔离。体内受体下调的一个重要机制是受体向内芽生进入腔内小泡，形成称为多囊泡体（MVBs）的特殊内体，随后与溶酶体融合，降解其内容物。这个过程需要四种异源寡聚蛋白复合物，统称为ESCRT机制。在酵母中，ESCRT-I是一种异源四聚体复合物，由三个保守亚基和第四个亚基组成，而在后生动物中缺乏可识别的同源物。利用秀丽隐杆线虫，我们鉴定出MVB-12，它是后生动物ESCRT-I的第四个亚基。MVB-12的缺失减缓了体内受体下调的动力学，但程度小于对其他ESCRT-I亚基的抑制。与这些发现一致，将MVB-12靶向膜需要其他ESCRT-I亚基，但将其余ESCRT-I组分靶向膜不需要MVB-12。内源性和重组ESCRT-I都是具有1:1:1:1亚基化学计量比的稳定复合物。MVB-12有两个人类同源物，它们与ESCRT-I组分TSG101共定位并共免疫沉淀。因此，MVB-12是后生动物ESCRT-I的一个保守核心组分，在MVB生物发生过程中调节其活性。
