Brieher W M, Yap A S, Gumbiner B M
Cellular Biochemistry and Biophysics Program, Memorial Sloan Kettering Cancer Center, New York 10021, USA.
J Cell Biol. 1996 Oct;135(2):487-96. doi: 10.1083/jcb.135.2.487.
Regulation of cadherin-mediated adhesion can occur rapidly at the cell surface. To understand the mechanism underlying cadherin regulation, it is essential to elucidate the homophilic binding mechanism that underlies all cadherin-mediated functions. Therefore, we have investigated the structural and functional properties of the extracellular segment of Xenopus C-cadherin using a purified, recombinant protein (CEC 1-5). CEC 1-5 supported adhesion of CHO cells expressing C-cadherin. The extracellular segment was also capable of mediating aggregation of microspheres. Chemical cross-linking and gel filtration revealed that CEC 1-5 formed dimers in the presence as well as absence of calcium. Analysis of the functional activity of purified dimers and monomers demonstrated that dimers retained substantially greater homophilic binding activity than monomers. These results demonstrate that lateral dimerization is necessary for homophilic binding between cadherin extracellular segments and suggest multiple potential mechanisms for the regulation of cadherin activity. Since the extracellular segment alone possessed significant homophilic binding activity, the adhesive activity of the extracellular segment in a cellular context was analyzed. The adhesion of CHO cells expressing a truncated version of C-cadherin lacking the cytoplasmic tail was compared to cells expressing the wild-type C-cadherin using a laminar flow assay on substrates coated with CEC 1-5. CHO cells expressing the truncated C-cadherin were able to attach to CEC 1-5 and to resist detachment by low shear forces, demonstrating that tailless C-cadherin can mediate basic, weak adhesion of CHO cells. However, cells expressing the truncated C-cadherin did not exhibit the complete adhesive activity of cells expressing wild-type C-cadherin. Cells expressing wild-type C-cadherin remained attached to CEC 1-5 at high shear forces, while cells expressing the tailless C-cadherin did not adhere well at high shear forces. These results suggest that there may be two states of cadherin-mediated adhesion. The first, relatively weak state can be mediated through interactions between the extracellular segments alone. The second strong adhesive state is critically dependent on the cytoplasmic tail.
钙黏蛋白介导的黏附调节可在细胞表面迅速发生。为了解钙黏蛋白调节的潜在机制，阐明所有钙黏蛋白介导功能所基于的嗜同性结合机制至关重要。因此，我们使用纯化的重组蛋白（CEC 1-5）研究了非洲爪蟾C-钙黏蛋白细胞外片段的结构和功能特性。CEC 1-5支持表达C-钙黏蛋白的CHO细胞的黏附。细胞外片段也能够介导微球的聚集。化学交联和凝胶过滤显示，CEC 1-5在有钙和无钙的情况下均形成二聚体。对纯化的二聚体和单体的功能活性分析表明，二聚体比单体保留了显著更高的嗜同性结合活性。这些结果表明，侧向二聚化对于钙黏蛋白细胞外片段之间的嗜同性结合是必要的，并提示了钙黏蛋白活性调节的多种潜在机制。由于单独的细胞外片段具有显著的嗜同性结合活性，因此分析了细胞外片段在细胞环境中的黏附活性。使用层流测定法，在包被有CEC 1-5的底物上，将表达缺失细胞质尾的截短型C-钙黏蛋白的CHO细胞的黏附与表达野生型C-钙黏蛋白的细胞进行比较。表达截短型C-钙黏蛋白的CHO细胞能够附着于CEC 1-5，并能抵抗低剪切力的分离，表明无尾C-钙黏蛋白可介导CHO细胞的基本、弱黏附。然而，表达截短型C-钙黏蛋白的细胞未表现出表达野生型C-钙黏蛋白的细胞的完全黏附活性。表达野生型C-钙黏蛋白的细胞在高剪切力下仍附着于CEC 1-5，而表达无尾C-钙黏蛋白的细胞在高剪切力下黏附不佳。这些结果表明，钙黏蛋白介导的黏附可能存在两种状态。第一种，相对较弱的状态可通过单独的细胞外片段之间的相互作用介导。第二种强黏附状态严重依赖于细胞质尾。
