Phosphorylation of smg p21B/rap1B p21 by cyclic GMP‐dependent protein kinase

Abstract smg p21B/ rap 1B p21, a member of ras p21-like small GTP-binding protein superfamily, has been shown to be phosphorylated by cyclic AMP-dependent protein kinase (protein kinase A). We show here that this protein was also phosphorylated by cyclic GMP-dependent protein kinase (protein kinase G) in a cell-free system. The same serine residue (Ser 179 ) in the C-terminal region was phosphorylated by both protein kinases G and A. The K m and V max values of smg p21B for protein kinase G were 5 × 10 −7 M and 4 × 10 −9 mol/min/mg, and those values for protein kinase A were 1 × 10 −7 M and 3 × 10 −8 mol/min/mg.