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

Abstract
smg p21B/rap1B 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 (Ser179) 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.