The HR1 repeat was first described as a three times repeated homology region of the N-terminal non-catalytic part of protein kinase PRK1(PKN) [1]. The first two of these repeats were later shown to bind the small G protein rho [2, 3] known to activate PKN in its GTP-bound form. Similar rho-binding domains also occur in a number of other protein kinases and in the rho-binding proteins rhophilin and rhotekin. Recently, the structure of the N-terminal HR1 repeat complexed with RhoA has been determined by X-ray crystallography [4]. It forms an antiparallel coiled-coil fold termed an ACC finger.

1: Cloning and expression patterns of two members of a novel protein-kinase-C-related kinase family. Palmer RH, Ridden J, Parker PJ; Eur J Biochem. 1995;227:344-351. PMID:7851406

2: Characterization of the interaction between RhoA and the amino-terminal region of PKN. Shibata H, Mukai H, Inagaki Y, Homma Y, Kimura K, Kaibuchi K, Narumiya S, Ono Y; FEBS Lett. 1996;385:221-224. PMID:8647255

3: Multiple interactions of PRK1 with RhoA. Functional assignment of the Hr1 repeat motif. Flynn P, Mellor H, Palmer R, Panayotou G, Parker PJ; J Biol Chem. 1998;273:2698-2705. PMID:9446575

4: The structural basis of Rho effector recognition revealed by the crystal structure of human RhoA complexed with the effector domain of PKN/PRK1. Maesaki R, Ihara K, Shimizu T, Kuroda S, Kaibuchi K, Hakoshima T; Mol Cell. 1999;4:793-803. PMID:10619026