The serine/threonine kinase STK4 (MST1) catalyzes its own autophosphorylation as well as the phosphorylation of SAV1. These two reactions are annotated here as a single concerted process that takes place in a tetrameric complex containing two STK4 (MST1) subunits and two SAV1 subunits, based on the observations that STK4 (MST1) can catalyze both phosphorylation reactions in vitro, as well as the observations that each protein dimerizes and that STK4 (MST1) and SAV1 associate to form a complex. The order in which the various components associate, the stoichiometry of the complex ultimately formed, and the point(s) in this association process at which phosphoryltion occurs have not been established in vitro or in vivo, however (Callus et al. 2006; Creasy et al. 1996; Praskova et al. 2004).