Multiple Interactions of PRK1 with RhoA

PRK1 (PKN) is a serine/threonine kinase that has been shown to be activated by RhoA (Amano, M., Mukai, H., Ono, Y., Chihara, K., Matsui, T., Hamajima, Y., Okawa, K., Iwamatsu, A., and Kaibuchi, K. (1996) Science 271, 648– 650). Detailed analysis of the PRK1 region involved in RhoA binding has revealed that two homologous sequences within the HR1 domain (HR1a and HR1b) both bind to RhoA; the third repeat within this domain, HR1c, does not bind RhoA. The related HR1 motif is also found to confer RhoA binding activity to the only other fully cloned member of this kinase family, PRK2. Furthermore, the predictive value of this motif is established for an HR1a sequence derived from a Caenorhabditis elegans open reading frame encoding a protein kinase of unknown function. Interestingly, the HR1a and HR1b subdomains are shown to display a distinctive nucleotide dependence for RhoA binding. HRIa is entirely GTP-dependent, while HR1b binds both GTPand GDP-bound forms of RhoA. This distinction indicates that there are two sites of contact between RhoA and PRK1, one contact through a region that is conformationally dependent upon the nucleotide-bound state of RhoA and one that is not. Analysis of binding to Rho/Rac chimera provides evidence for a HR1a but not HR1b interaction in the central third of Rho. Additionally, it is observed that the V14RhoA mutant binds HR1a but does not bind HR1b. This distinct binding behavior corroborates the conclusion that there are independent contacts on RhoA for the HR1a and HR1b motifs.