Kinases, structure-function correlations

Src is the prototype of the superfamily of protein tyrosine kinases and was one of the first protein kinases to be characterized by various genetic, cellular, and structure-function studies to help imderstand its role in signal transduction pathways as well as in disease processes, including cancer, osteoporosis, and both tumor- and inflammation-mediated bone loss [28-38]. In fact, studies on Src provided some of the first evidence correlating protein kinase activity and substrate protein phosphorylation in the regulation of signal transduction pathways relative to normal cellular activity as well as mahgnant transformations. Src family kinases include Fyn, Yes, Yrk, Blk, Fgr, Hck, Lyn,... 

Cysteine-scanning mutagenesis, involving more than 100 mutations, has been systematically carried out through Cl—C3, the cytoplasmic terminations of TM1-TM7, H8, and the C-terminal tail. In addition, more than 40 pairs of cysteines have been introduced at the cytoplasmic face. With these mutants as a basis set, three classes of experiments have been carried out, namely SDSL, sulfhydryl reactivity, and disulfide cross-linking kinetics. A global comparison of the results provides a unique view of the solution state, its dynamics, and its correlation with the crystal structure. By solution state is meant, in all cases, rhodopsin solubilized in dodecyl maltoside (DM) micelles. The measured functional properties of rhodopsin, namely transducin activation (Resek et al., 1993) and phosphorylation by the rhodopsin kinase (Thurmond et al., 1997), are conserved in this detergent, and it is presumed to be a reasonable approximation to the bilayer environment. 

In principle, one can classify the phosphoproteins into two groups (1) functional, whose phosphorylation is correlated with contraction, and (2) structural, whose phosphate content remains rather steady during the contraction cycle. Structural phosphoproteins could make contact with other proteins to form a specific protein network. Alternatively, they may bind divalent metals, Ca + or Mg2+. The common experience of the slow turnover of phosphate in these proteins also suggests that the covalently bound phosphate is not free and, therefore, not readily available for protein kinases and phosphatases. Future investigation should provide information about the role of the structural phosphoproteins in smooth muscle. 

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