Parker C M, Pujades C, Brenner M B, Hemler M E
Brigham and Womens Hospital, Department of Rheumatology and Immunology, Boston, Massachusetts.
J Biol Chem. 1993 Apr 5;268(10):7028-35.
The integrin alpha 4 beta 1 (VLA-4) is a versatile cell-cell and cell-extracellular matrix adhesion receptor. The alpha 4 subunit can be expressed on the cell surface in two forms: an intact form (alpha 4/150) and a cleaved form (alpha 4/80,70). Here we have characterized a third form of alpha 4, called alpha 4/180. Although alpha 4/180 (M(r) 180, nonreduced) is different in size than alpha 4/150 (M(r) 150, nonreduced), these two forms are clearly related, as they (i) shared the same amino-terminal sequence, (ii) were both recognized in Western blots by an anti-alpha 4 COOH-terminal antiserum, (iii) migrated with the same apparent size and charge when reduced, and (iv) were both immunoprecipitated using anti-VLA-4 reagents. In pulse-chase experiments, precursors to both forms appeared simultaneously and matured at the same rate, indicating that one is most likely not the biosynthetic precursor of the other. Although reduction of alpha 4/180 to yield alpha 4/150 suggested the release of a cysteine-linked 30-kDa fragment, seven different biochemical techniques failed to identify such a fragment. Also, alpha 4/180 was converted to alpha 4/150 by incubation at pH 11, by treatment with EDTA at 56 degrees C, or by heating in the presence of elevated SDS levels. Together our findings suggest that alpha 4/180 and alpha 4/150 represent different conformations of the same alpha 4 polypeptide, with the former having anomalous slower migration in SDS-polyacrylamide gel electrophoresis. This unusual biochemical feature of alpha 4 is not shared by other beta 1-associated integrin alpha subunits and suggests that VLA-4 has unique structural properties.
整合素α4β1（VLA - 4）是一种多功能的细胞 - 细胞及细胞 - 细胞外基质黏附受体。α4亚基可以以两种形式表达在细胞表面：完整形式（α4/150）和裂解形式（α4/80,70）。在此我们鉴定出了α4的第三种形式，称为α4/180。尽管α4/180（非还原状态下分子量为180）在大小上与α4/150（非还原状态下分子量为150）不同，但这两种形式明显相关，因为它们：（i）共享相同的氨基末端序列；（ii）在蛋白质免疫印迹中都能被抗α4羧基末端抗血清识别；（iii）还原后迁移的表观大小和电荷相同；（iv）都能用抗VLA - 4试剂进行免疫沉淀。在脉冲追踪实验中，两种形式的前体同时出现并以相同速率成熟，这表明一种很可能不是另一种的生物合成前体。尽管将α4/180还原产生α4/150提示释放了一个与半胱氨酸相连的30 kDa片段，但七种不同的生化技术都未能鉴定出这样一个片段。此外，通过在pH 11下孵育、在56℃用EDTA处理或在高SDS水平下加热，α4/180可转化为α4/150。我们的研究结果共同表明，α4/180和α4/150代表同一α4多肽的不同构象，前者在SDS - 聚丙烯酰胺凝胶电泳中迁移异常缓慢。α4的这种不寻常的生化特征在其他与β1相关的整合素α亚基中并未出现，这表明VLA - 4具有独特的结构特性。
