()

G 7

()

() Ser/Thr Tyr

() GTP G G GTP GDP GTP

()

()

SH2 (Src homology 2 domain) 100pYSH3(Src homology 3 domain) PXXP

PH (pleckstrin homology) 120500PH(PI-4,5-P2PI-3,4-P2PI-3,4,5-P3,C-PHG

Regulation of cellular proteins by phosphorylation. Protein kinases transfer phosphate groups (Pi) from ATP to serine, threonine, or tyrosine residues on substrate proteins. This phosphorylation reversibly alters the structure and function of cellular proteins. Removal of the phosphate groups is catalyzed by protein phosphatases. Both kinases and phosphatases are regulated by a variety of intracellular second messengers.