CALCIUM-BINDING DOMAINS AND CALCIUM-INDUCED CONFORMATIONAL TRANSITION OF SPARC-BM-40-OSTEONECTIN, AN EXTRACELLULAR GLYCOPROTEIN EXPRESSED IN MINERALIZED AND NONMINERALIZED TISSUES
CALCIUM-BINDING DOMAINS AND CALCIUM-INDUCED CONFORMATIONAL TRANSITION OF SPARC-BM-40-OSTEONECTIN, AN EXTRACELLULAR GLYCOPROTEIN EXPRESSED IN MINERALIZED AND NONMINERALIZED TISSUES
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DOI：
10.1021/bi00396a015
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发表时间：
1987-11-03
期刊：
影响因子：
2.9
通讯作者：
HOGAN, B
中科院分区：
文献类型：
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作者：
ENGEL, J;TAYLOR, W;HOGAN, B
SPARC， BM-40和骨连接蛋白是相同或非常密切相关的细胞外蛋白，表观Mr为43,000（从序列预测Mr为33,000）。它们最初分别从壁内胚层细胞、基底膜产生肿瘤细胞和骨中分离出来，但在各种组织中分布相当广泛。鉴于骨连接蛋白的钙结合活性，我们分析了SPARC序列，发现了两个可能的钙结合域。一个是带有谷氨酸残基簇的n端酸性区域。这个区域，虽然不是。-羧基化的，不是同源的，类似于。-羧谷氨酸（Gla）结构域影响凝血系统中维生素K依赖蛋白的电荷密度、带负电荷簇的大小以及通过富含半胱氨酸的结构域与分子其余部分的连接。另一个区域是位于c端附近的EF-hand钙结合域。利用已知的肠钙结合蛋白坐标对EF-hand结构进行建模，预测了E和F螺旋之间的二硫键。二硫桥显然用于稳定细胞外蛋白中分离的钙环。正如观察到的细胞质中含有ef -hand的蛋白质和含有Gla结构域的蛋白质以及含有Gla结构域的蛋白质，一个主要的构象转变是在几个Ca2+离子结合后在BM-40中诱导的。这伴随着-螺旋度增加35%。在220 nm处，圆二色性信号对钙浓度的依赖性呈显著的s型性，表明该过程是协同的。鉴于SPARC/BM-40/骨连接蛋白的性质、丰度和广泛分布，我们认为它在细胞外基质的钙依赖过程中具有相当普遍的调节功能。
SPARC, BM-40, and osteonectin are identical or very closely related extracellular proteins of apparent Mr 43,000 (Mr 33,000 predicted from sequence). They were originally isolated from parietal endoderm cells, basement membrane producing tumors, and bone, respectively, but are rather widely distributed in various tissues. In view of the calcium binding activity reported for osteonectin, we analyzed the SPARC sequence and found two putative calcium binding domains. One is an N-terminal acidic region with clusters of glutamic acid residues. This region, although neither .gamma.-carboxylated nor homologous, resembles the .gamma.-carboxyglutamic acid (Gla) domain of vitamin K dependent proteins of the blood clotting system in charge density, size of negatively charged clusters, and linkage to the rest of the molecule by a cysteine-rich domain. The other region is an EF-hand calcium binding domain located near the C-terminus. A disulfide bond between the E and F helix is predicted from modeling the EF-hand structure with the known coordinates of intestinal calcium binding protein. The disulfide bridge apparently serves to stabilize the isolated calcium loop in the extracellular protein. As observed for cytoplasmic EF-hand-containing proteins and for Gla domain containing proteins and for Gla domain containing proteins, a major conformational transition is induced in BM-40 upon binding of several Ca2+ ions. This is accompanied by a 35% increase in .alpha.-helicity. A pronounced sigmoidicity of the dependence of the circular dichroism signal at 220 nm on calcium concentration indicates that the process is cooperative. In view of its properties, abundance, and wide distribution, it is proposed that SPARC/BM-40/osteonectin has a rather general regulatory function in calcium-dependent processes of the extracellular matrix.