Bhattacharya G, Boman B M
Creighton Cancer Center, Creighton University, Omaha, NE 68178.
Biochem Biophys Res Commun. 1995 Mar 8;208(1):103-10. doi: 10.1006/bbrc.1995.1311.
The adenomatous polyposis coli (APC) gene is etiologically associated with familial adenomatous polyposis and gastrointestinal malignancies, but its cellular function and role in tumorigenesis are unclear. Recent reports indicate that wild-type, but not mutant, APC gene product (APC) is associated with and promotes the assembly of cytoskeletal microtubules in vitro, suggesting that this mechanism has importance in tumor development. Because other microtubule-associated proteins (MAPs) undergo phosphorylation in their normal functioning, we postulated that APC is a phosphoprotein. HCT116 cells, containing full-length APC protein, were [32P]-prelabeled, and a 300-kDa band corresponding to phosphorylated APC was immunoprecipitated using each of three different anti-APC antibodies. High voltage electrophoresis of [32P]-labeled APC showed the presence of phospho-serine and phospho-threonine residues. Further immunoprecipitation analyses showed phosphorylation of i) full-length APC in human lymphoblastoid cells and ii) carboxyl-truncated APC in SW480 and DiFi colon carcinoma cells. Thus, APC is probably a phosphoprotein in normal and malignant tissues. We hypothesize a mechanism whereby phosphorylation of APC may play a regulatory role in its interaction with microtubules. This may involve phosphorylation of (Ser/Thr)-Pro amino acid motifs in APC's basic domain. We propose that deletion of this domain disrupts APC binding to microtubules, explaining how APC mutations are linked to cancer development.
腺瘤性结肠息肉病（APC）基因在病因学上与家族性腺瘤性息肉病和胃肠道恶性肿瘤相关，但其细胞功能及在肿瘤发生中的作用尚不清楚。最近的报告表明，野生型而非突变型的APC基因产物（APC）在体外与细胞骨架微管的组装相关并促进其组装，提示该机制在肿瘤发展中具有重要意义。由于其他微管相关蛋白（MAPs）在其正常功能过程中会发生磷酸化，我们推测APC是一种磷蛋白。对含有全长APC蛋白的HCT116细胞进行[32P]预标记，然后使用三种不同的抗APC抗体分别免疫沉淀出一条对应于磷酸化APC的300 kDa条带。对[32P]标记的APC进行高压电泳显示存在磷酸丝氨酸和磷酸苏氨酸残基。进一步的免疫沉淀分析表明：i）人淋巴母细胞中的全长APC发生了磷酸化，以及ii）SW480和DiFi结肠癌细胞中的羧基截短型APC发生了磷酸化。因此，APC在正常组织和恶性组织中可能都是一种磷蛋白。我们推测了一种机制，即APC的磷酸化可能在其与微管的相互作用中发挥调节作用。这可能涉及APC碱性结构域中（丝氨酸/苏氨酸）-脯氨酸氨基酸基序的磷酸化。我们提出，该结构域的缺失会破坏APC与微管的结合，这解释了APC突变与癌症发展之间的联系。
