Interaction between the protein kinase B-Raf and the alpha-subunit of the 11S proteasome regulator.

Protein kinases of the Raf family act as signal-transducing elements downstream of activated cell surface receptors and are involved in the regulation of proliferation, differentiation, and cell survival. Whereas the role of c-Raf-1 as a mitogen-activated protein/extracellular signal-regulated kinase activator within the mitogenic cascade is well established, less is known about the mammalian Raf isoforms A-Raf and B-Raf. Here we report that B-Raf binds to PA28alpha, one of two subunits of the 11S regulator of proteasomes. PA28alpha was isolated as a B-Raf-binding protein in a yeast two-hybrid screen of a PC12 cDNA library. Both proteins can be coimmunoprecipitated after transient expression in 293 cells. No association could be found between PA28alpha and A-Raf or c-Raf-1. B-Raf binds to a region in PA28alpha that is important for its proteasome-activating function.