Balzac F, Belkin A M, Koteliansky V E, Balabanov Y V, Altruda F, Silengo L, Tarone G
Department of Genetics, Biology and Medical Chemistry, University of Torino, Italy.
J Cell Biol. 1993 Apr;121(1):171-8. doi: 10.1083/jcb.121.1.171.
We have previously described a variant form of the integrin beta 1 subunit (beta 1B)1 characterized by an altered sequence at the cytoplasmic domain. Using polyclonal antibodies to a synthetic peptide corresponding to the unique sequence of the beta 1B, we analyzed the expression of this molecule in human tissues and cultured cells. Western blot analysis showed that the beta 1B is expressed in skin and liver and, in lower amounts, in skeletal and cardiac muscles. The protein was not detectable in brain, kidney, and smooth muscle. In vitro cultured keratinocytes and hepatoma cells are positive, but fibroblasts, endothelial cells, and smooth muscle cells are negative. An astrocytoma cell line derived from immortalized fetal astrocytes was found to express beta 1B. In these cells beta 1B represent integral of 30% of the beta 1 and form heterodimers with alpha 1 and alpha 5 subunits. To investigate the functional properties of beta 1B, the full-length cDNA coding for this molecule was transfected into CHO cells. Stable transfectants were selected and the beta 1B was identified by a mAb that discriminate between the transfected human protein and the endogenous hamster beta 1A. Immunoprecipitation experiments indicated that the beta 1B was exported at the cell surface in association with the endogenous hamster alpha subunits. The alpha 5/beta 1B complex bound to a fibronectin-affinity matrix and was specifically released by RGD-containing peptides. Thus beta 1B and beta 1A are similar as far as the alpha/beta association and fibronectin binding are concerned. The two proteins differ, however, in their subcellular localization. Immunofluorescence studies indicated, in fact, that beta 1B, in contrast to beta 1A, does not localize in focal adhesions. The restricted tissue distribution and the distinct subcellular localization, suggest that beta 1B has unique functional properties.
我们之前描述过整联蛋白β1亚基的一种变体形式（β1B），其特征在于胞质结构域的序列发生了改变。我们使用针对与β1B独特序列相对应的合成肽的多克隆抗体，分析了该分子在人体组织和培养细胞中的表达情况。蛋白质印迹分析表明，β1B在皮肤和肝脏中表达，在骨骼肌和心肌中表达量较低。在脑、肾和平滑肌中未检测到该蛋白。体外培养的角质形成细胞和肝癌细胞呈阳性，但成纤维细胞、内皮细胞和平滑肌细胞呈阴性。发现源自永生化胎儿星形胶质细胞的星形细胞瘤细胞系表达β1B。在这些细胞中，β1B占β1总量的30%，并与α1和α5亚基形成异二聚体。为了研究β1B的功能特性，将编码该分子的全长cDNA转染到CHO细胞中。筛选出稳定转染子，并用一种能区分转染的人蛋白和内源性仓鼠β1A的单克隆抗体鉴定β1B。免疫沉淀实验表明，β1B与内源性仓鼠α亚基一起转运到细胞表面。α5/β1B复合物与纤连蛋白亲和基质结合，并被含RGD的肽特异性释放。因此，就α/β缔合和纤连蛋白结合而言，β1B和β1A是相似的。然而，这两种蛋白在亚细胞定位上有所不同。事实上，免疫荧光研究表明，与β1A不同，β1B并不定位于粘着斑。有限的组织分布和独特的亚细胞定位表明，β1B具有独特的功能特性。
