Structure and function of C-CAM1: effects of the cytoplasmic domain on cell aggregation.
Structure and function of C-CAM1: effects of the cytoplasmic domain on cell aggregation.
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C-CAM1 的结构和功能：胞质结构域对细胞聚集的影响。
DOI：
10.1042/bj3110239
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发表时间：
1995
期刊：
影响因子：
--
通讯作者：
Hixson,DC
中科院分区：
文献类型：
--
作者：
Lin,SH;Luo,W;Earley,K;Cheung,P;Hixson,DC
C-CAM是免疫球蛋白超基因家族的上皮细胞黏附分子，其序列与癌胚抗原高度同源。C-CAM及其人类同源物胆汁糖蛋白在CEA家族蛋白中是独一无二的，因为它们具有细胞质结构域。此外，选择性剪接产生具有不同细胞质结构域的C-CAM亚型，这表明C-CAM的细胞质结构域可能在调节C-CAM的功能中发挥重要作用。通过使用正义和反义方法，我们已经证明C-CAM1在前列腺癌的发生中是一种肿瘤抑制因子。这一观察结果提出了C-CAM1的胞浆结构域可能参与信号转导或与细胞骨架元件相互作用从而引发肿瘤抑制功能的可能性。C-CAM1的胞浆结构域包含几个潜在的磷酸化位点，包括假定的环磷酸腺苷依赖的激酶和酪氨酸激酶的共同序列。潜在的酪氨酸磷酸化位点之一位于抗原受体同源(ARH)结构域。膜结合的IgM分子的ARH结构域是B细胞信号转导所必需的。这些结构特征表明，C-CAM1的胞质结构域可能在信号转导中起重要作用。为了测试这种可能性，我们产生了几个定点C-CAM1突变体，并在体内测试了它们支持黏附和被磷酸化的能力。这些研究结果表明，Tyr-488在体内是磷酸化的。然而，用苯丙氨酸取代这种酪氨酸并没有显著影响其黏附功能。同样，丝氨酸和苏氨酸残基在体内被磷酸化，但潜在的环状AMP依赖的激酶位点的缺失并没有显著降低黏附功能。这些结果表明，C-CAM1胞浆结构域中的激酶磷酸化位点不是黏附功能所必需的。然而，这些磷酸化位点可能参与了C-CAM介导的信号转导的调节。因此，可能对C-CAM的黏附和信号转导功能有不同的结构要求。顺便说一句，含有10个氨基酸的细胞质结构域的C-CAM1缺失突变体能够支持黏附活性。这与我们之前的发现相反，带有6个氨基酸的细胞质结构域的C-CAM亚型C-CAM3不能支持细胞黏附。这一结果表明，C-CAM3中没有的另外四个氨基酸含有潜在的Ser/Thr磷酸化位点，对黏附功能是重要的。
C-CAMs are epithelial cell-adhesion molecules of the immunoglobulin supergene family with sequences highly homologous to carcinoembryonic antigen (CEA). C-CAMs and their human homologues, biliary glycoproteins, are unique among the CEA-family proteins in that they have cytoplasmic domains. Furthermore, alternative splicing generates C-CAM isoforms with different cytoplasmic domains, suggesting that the cytoplasmic domains of C-CAM may play important roles in regulating the function or functions of C-CAM. By using both sense and antisense approaches, we have shown that C-CAM1 is a tumour suppressor in prostate carcinogenesis. This observation raises the possibility that the cytoplasmic domain of C-CAM1 may be involved in signal transduction or interaction with cytoskeletal elements to elicit the tumour suppressor function. The cytoplasmic domain of C-CAM1 contains several potential phosphorylation sites, including putative consensus sequences for cyclic AMP-dependent kinase and tyrosine kinase. One of the potential tyrosine phosphorylation sites is located within the antigen-receptor homology (ARH) domain. The ARH domain of the membrane-bound IgM molecule is necessary for signal transduction in B-cells. These structural features suggest that the cytoplasmic domain of C-CAM1 may be important for signal transduction. To test this possibility, we generated several site-directed C-CAM1 mutants and tested their ability to support adhesion and their abilities to be phosphorylatedin vivo. Results from these studies revealed that Tyr-488 is phosphorylatedin vivo. However, replacing this tyrosine with phenylalanine did not significantly compromise its adhesion function. Similarly, Ser and Thr residues are phosphorylatedin vivo, but deletion of the potential cyclic AMP-dependent kinase site did not significantly reduce the adhesion function. These results suggest that the kinase phosphorylation sites in the cytoplasmic domain of C-CAM1 are not required for the adhesion function. However, these phosphorylation sites are probably involved in the regulation of C-CAM-mediated signal transduction. Thus, there are probably distinct structural requirements for the adhesion and the signal transduction functions of C-CAM. Incidentally, a C-CAM1 deletion mutant containing a 10-amino-acid cytoplasmic domain was able to support adhesion activity. This is in contrast to our previous finding that a C-CAM isoform, C-CAM3, with a 6-amino-acid cytoplasmic domain could not support cell adhesion. This result indicates that the extra four amino acids, which are absent in C-CAM3 and contain a potential Ser/Thr phosphorylation site, are important for the adhesion function.