Arneson M L, Cheng H L, Louis C F
Department of Biochemistry, University of Minnesota, St Paul 55108, USA.
Eur J Biochem. 1995 Dec 1;234(2):670-9. doi: 10.1111/j.1432-1033.1995.670_b.x.
The cAMP-dependent protein-kinase-catalyzed phosphorylation of the two major intrinsic lens fiber cell plasma membrane proteins, MP20 and MP26, is likely restricted to the inner cortical and nuclear regions of the lens in vivo. The ovine-lens-specific connexin, MP70, that has been identified as Cx50 in mice and Cx45.6 in the chick, is also a protein kinase substrate although it does not appear to be phosphorylated by a number of protein kinases including cAMP-dependent protein kinase, calmodulin-dependent protein kinase or protein kinase C. Rather, an extrinsic lens membrane fraction was isolated which contained protein kinase activity that catalyzed the phosphorylation of MP70; this protein kinase activity was cAMP-independent, Ca(2+)-independent, Mg(2+)-dependent, phosphorylated MP70 on a serine residue(s) and migrated with a molecular mass of 35 kDa on a gel filtration column. Both MP70 phosphorylation and the endogenous protein kinase activity were restricted to the lens outer cortical region. This membrane-associated protein kinase activity represents the first reported partial characterization of an endogenous lens fiber cell protein kinase activity that catalyzes the phosphorylation of a lens connexin protein. The phosphatase-induced shift in the electrophoretic mobility of MP70 is not reversed by this protein kinase, indicating that MP70 is likely phosphorylated on different residues by two or more protein kinases.
环磷酸腺苷（cAMP）依赖性蛋白激酶催化的晶状体纤维细胞两大主要内在质膜蛋白MP20和MP26的磷酸化，在体内可能仅限于晶状体的内皮质和核区域。绵羊晶状体特异性连接蛋白MP70，在小鼠中被鉴定为Cx50，在鸡中被鉴定为Cx45.6，它也是一种蛋白激酶底物，尽管它似乎不会被包括cAMP依赖性蛋白激酶、钙调蛋白依赖性蛋白激酶或蛋白激酶C在内的多种蛋白激酶磷酸化。相反，分离出一种晶状体外膜组分，其含有催化MP70磷酸化的蛋白激酶活性；这种蛋白激酶活性不依赖于cAMP，不依赖于Ca(2+)，依赖于Mg(2+)，在丝氨酸残基上磷酸化MP70，并且在凝胶过滤柱上以35 kDa的分子量迁移。MP70磷酸化和内源性蛋白激酶活性都仅限于晶状体外皮质区域。这种与膜相关的蛋白激酶活性代表了首次报道的对一种内源性晶状体纤维细胞蛋白激酶活性的部分特性描述，该活性催化晶状体连接蛋白的磷酸化。MP70电泳迁移率的磷酸酶诱导变化不会被这种蛋白激酶逆转，这表明MP70可能被两种或更多种蛋白激酶磷酸化在不同的残基上。
