High molecular weight muscle protein(s), present as a "doublet" (approximately 320 and approximately 290 kDa apparent molecular weight) in partially purified preparations of inhibitor-2 from rabbit skeletal muscle, and homogeneous bovine brain microtubule associated protein-2 are both in vitro substrates for a soluble insulin-stimulatable serine/threonine kinase in 3T3-L1 adipocytes. The high molecular weight muscle substrate "doublet" was specifically immunoprecipitated by affinity-purified anti-microtubule associated protein-2 antibody.