The second messenger guanosine 3′,5′-monophosphate (cGMP) controls a broad range of cellular functions, many of which are regulated through the cGMP-dependent protein kinase. Cyclic GMP is synthesized by guanylyl cyclases (GCs) that catalyze their formation from guanosine triphosphate. A study by Guo et al. provides insight into the upstream regulation of the GCs. Unlike the adenyl cyclases, which are well known as targets of GPCRs [heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors], transmembrane guanylyl cyclases are thought to act as receptors themselves, although not all have identified ligands. Guo et al. explored how intracellular signals can also regulate the transmembrane guanylyl cyclase GC-E and observed that transient overexpression of the small GTPase Rac1 led to activation of GC-E. Activation of the platelet-derived growth factor receptor (PDGFR) also stimulated formation of cGMP, and this was inhibited when the authors used a dominant-negative form of Rac1 to inhibit activity of endogenous Rac1. The authors tested known targets of Rac1 to explore how Rac1 might be coupled to activation of GC-E. PAK (p21-activated protein kinase) is one such target, and it could substitute for Rac1 in activating formation of cGMP; expression of the autoinhibitory domain of PAK also inhibited the effect of constitutively active Rac1 to stimulate GC-E activity. The authors used in vitro and in vivo studies with purified proteins and protein fragments to show that PAK directly interacts with and stimulates GC-E. This activation required the kinase activity of PAK--but apparently only for autophosphorylation, because GC-E was not phosphorylated by PAK. This new pathway from receptor tyrosine kinase to cGMP was shown to be physiologically important in the control of cell migration. Depletion of GC-A in mouse embryo fibroblasts with small interfering RNA (siRNA) reduced the effect of PDGF on accumulation of cGMP and inhibited PDGF-dependent cell migration and formation of lamellipodia. Settleman discusses the work in a commentary and emphasizes some of the intriguing questions raised by the work, such as why Cdc42, another small GTPase that also activates PAK, fails to activate GC-E.
D. Guo, Y.-C. Tan, D. Wang, K. S. Madhusoodanan, Y. Zheng, T. Maack, J. J. Zhang, X.-Y. Huang, A Rac-cGMP signaling pathway. Cell 128, 341-355 (2007). [Online Journal]
J. Settleman, PAK-in’ up cGMP for the move. Cell 128, 237-238 (2007). [Online Journal]