ABSTRACT
The ubiquitous enzymes which are operationally termed "casein kinases" (CK) were the first protein kinases to be detected, as early as in 1954. Although a thorough characterization of the different subsets of casein kinases was achieved, some of their distinctive features were anticipated by pioneer studies in the late 1960s and early 1970s. A recurrent feature of CK-2 is a relatively large molecular weight as compared to that of CK-1, a property which is often exploited for separating these two types of enzymes by gel filtration. Until the end of the 1970s the only known phosphorylatable substrates of c-2 were foreign proteins like casein fractions and phosvitin, obviously unrelated to this enzyme. As long as its physiological targets remained unknown, the specificity of CK-2 was necessarily explored with only the aid of foreign protein substrates. Unlike CK-1, the mammary gland casein kinase display a site specificity partially overlapping that of CK-2.
