THE STE20-LIKE PROTEIN-KINASE, MST1, DIMERIZES AND CONTAINS AN INHIBITORY DOMAIN

The human serine/threonine protein kinases, Mst1 and Mst2, share consi derable homology to Ste20 and p21-activated kinase (Pak) throughout th eir catalytic domains. However, outside the catalytic domains there ar e no significant homologies to previously described Ste20-like kinases or other proteins. To understand the role of the nonhomologous region s, we performed a structure/function analysis of Mst1. A series of COO H-terminal and internal deletions indicates that there is an element w ithin a central 63-amino acid region of the molecule that inhibits kin ase activity. Removal of this domain increases kinase activity approxi mately 9-fold. Coimmunoprecipitation assays, the yeast two-hybrid proc edure, and in vitro cross-linking analysis indicate that Mst1 homodime rizes and that the extreme COOH-terminal 57 amino acids are required f or self-association. Size exclusion chromatography indicates that Mst1 is associated with a high molecular weight complex in cells, suggesti ng that other proteins may also oligomerize with this kinase. While lo ss of dimerization alone does not affect kinase activity, a molecule l acking both the dimerization and inhibitory domains is not as active a s one which lacks only the inhibitory domain. Comparison of Mst1 and M st2 indicates that both functional domains lie in regions conserved be tween the two molecules.