Signal transduction via protein phosphorylation

Historically, the protein phosphatases were classified into two groups according to their substrate specificity tyrosine phosphatases and serine/threonine protein phosphatases. The latter are metalloenzymes we will refer to these as metallophospha-tases, a superfamily of enzymes that includes the serine/threonine protein phosphatases, purple acid phosphatases, and other enzymes capable of phosphoryl group transfer reactions such as bacteriophage A protein phosphatase [17, 18]. 

The eukaryotic serine/threonine protein phosphatases were originally classified as either type 1 or 2 serine/threonine protein phosphatases based on their biochemical properties (Table 17-1) [19-21]. Type 1 protein phosphatases selectively dephos- 

Catalytic subunit PPlc (37kDa) PP2Ac (34-40 kDa) CNA(58kDa) PP2Cc (42-44 kDa) 

Divalent metal ion activators None None Ca /calmodulin  

A metal-dependent form of PP2A has been reported [47]  

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Redox regulation of signal transduction via protein phosphorylation mechanisms by calcineurin, a threonine/serine phosphatase, is the theme explored by Rusnak and colleagues (Chapter 17). Detailed site-directed mutagenesis and spectroscopic studies reveal the importance of active site amino acid residues and redox state of... 

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