Young P, Deveraux Q, Beal R E, Pickart C M, Rechsteiner M
Department of Biochemistry, University of Utah School of Medicine, Salt Lake City, Utah 84132, USA.
J Biol Chem. 1998 Mar 6;273(10):5461-7. doi: 10.1074/jbc.273.10.5461.
Ubiquitylated proteins are degraded by the 26 S protease, an enzyme complex that contains 30 or more unique subunits. One of these proteins, subunit 5a (S5a), has been shown to bind ubiquitin-lysozyme conjugates and free polyubiquitin chains. Using deletional analysis, we have identified in the carboxyl-terminal half of human S5a, two independent polyubiquitin binding sites whose sequences are highly conserved among higher eukaryotic S5a homologs. The sites are approximately 30-amino acids long and are separated by 50 intervening residues. When expressed as small fragments or when present in full-length S5a molecules, the sites differ at least 10-fold in their apparent affinity for polyubiquitin chains. Each binding site contains 5 hydrophobic residues that form an alternating pattern of large and small side chains, e.g. Leu-Ala-Leu-Ala-Leu, and this pattern is essential for binding ubiquitin chains. Based on the importance of the alternating hydrophobic residues in the binding sites and previous studies showing that a hydrophobic patch on the surface of ubiquitin is essential for proteolytic targeting, we propose a model for molecular recognition of polyubiquitin chains by S5a.
泛素化蛋白由26S蛋白酶降解，这是一种含有30个或更多独特亚基的酶复合物。其中一种蛋白，即亚基5a（S5a），已被证明能结合泛素-溶菌酶缀合物和游离的多聚泛素链。通过缺失分析，我们在人S5a的羧基末端一半区域鉴定出两个独立的多聚泛素结合位点，其序列在高等真核生物S5a同源物中高度保守。这些位点大约有30个氨基酸长，中间相隔50个间隔残基。当以小片段形式表达或存在于全长S5a分子中时，这些位点对多聚泛素链的表观亲和力至少相差10倍。每个结合位点包含5个疏水残基，它们形成大小侧链交替的模式，例如亮氨酸-丙氨酸-亮氨酸-丙氨酸-亮氨酸，这种模式对于结合泛素链至关重要。基于结合位点中交替疏水残基的重要性以及先前的研究表明泛素表面的疏水斑块对于蛋白水解靶向至关重要，我们提出了一个S5a对多聚泛素链进行分子识别的模型。
