Patel H V, Tzeng S R, Liao C Y, Chen S H, Cheng J W
Department of Life Science, National Tsing Hua University, Hsinchu, Taiwan.
Proteins. 1997 Dec;29(4):545-52. doi: 10.1002/(sici)1097-0134(199712)29:4<545::aid-prot13>3.0.co;2-m.
X-linked agammaglobulinemia (XLA), an inherited disease, is caused by mutations in the Bruton's tyrosine kinase (BTK). The absence of functional BTK leads to failure of B-cell differentiation; this incapacitates antibody production in XLA patients, who suffer from recurrent, sometimes lethal, bacterial infections. BTK plays an important role in B-cell development; it interacts with several proteins in the context of signal transduction. Point mutation in the BTK gene that leads to deletion of C-terminal 14 aa residues of BTK SH3 domain was found in a patient family. To understand the role of BTK, we studied binding of BTK SH3 domain (aa 216-273, 58 residues) and truncated SH3 domain (216-259, 44 residues) with proline-rich peptides; the first peptide constitutes the SH3 domain of BTK, while the latter peptide lacks 14 amino acid residues of the C terminal. Proline-rich peptides selected from TH domain of BTK and p120cbl were studied. It is known that BTK TH domain binds to SH3 domains of various proteins. We found that BTK SH3 domain binds to peptides of BTK TH domain. This suggests that BTK SH3 and TH domains may associate in inter- or intramolecular fashion, which raises the possibility that the kinase may be regulating its own activity by restricting the availability of both its ligand-binding modules. We also found that truncated SH3 domain binds to BTK TH domain peptide less avidly than does normal SH3 domain. Also, we show that the SH3 and truncated SH3 domains bind to peptide of p120cbl, but the latter domain binds weakly. It is likely that the truncated SH3 domain fails to present to the ligand the crucial residues in the correct context, hence the weaker binding. These results delineate the importance of C-terminal in binding of SH3 domains and indicate also that improper folding and the altered binding behavior of mutant BTK SH3 domain likely leads to XLA.
X连锁无丙种球蛋白血症（XLA）是一种遗传性疾病，由布鲁顿酪氨酸激酶（BTK）的突变引起。功能性BTK的缺失导致B细胞分化失败；这使得XLA患者无法产生抗体，患者会遭受反复的、有时甚至是致命的细菌感染。BTK在B细胞发育中起重要作用；它在信号转导过程中与多种蛋白质相互作用。在一个患者家族中发现了BTK基因的点突变，该突变导致BTK SH3结构域的C末端14个氨基酸残基缺失。为了了解BTK的作用，我们研究了BTK SH3结构域（第216 - 273位氨基酸，共58个残基）和截短的SH3结构域（216 - 259位氨基酸，共44个残基）与富含脯氨酸的肽段的结合；第一个肽段构成BTK的SH3结构域，而后者肽段缺少C末端的14个氨基酸残基。我们研究了从BTK的TH结构域和p120cbl中筛选出的富含脯氨酸的肽段。已知BTK的TH结构域与多种蛋白质的SH3结构域结合。我们发现BTK的SH3结构域与BTK的TH结构域的肽段结合。这表明BTK的SH3和TH结构域可能以分子间或分子内的方式相互作用，这增加了激酶可能通过限制其两个配体结合模块的可用性来调节自身活性的可能性。我们还发现截短的SH3结构域与BTK的TH结构域肽段的结合不如正常SH3结构域紧密。此外，我们表明SH3和截短的SH3结构域都与p120cbl的肽段结合，但后者结构域的结合较弱。截短的SH3结构域可能无法在正确的环境中向配体呈现关键残基，因此结合较弱。这些结果阐明了C末端在SH3结构域结合中的重要性，也表明突变的BTK SH3结构域的错误折叠和改变的结合行为可能导致XLA。
