MAP4K4 associated with one or more striatins in cardiomyocytes, potentially through a coiled-coil domain (CCD) in MAP4K4(351–495). We identified an additional helix structure (KH), C-terminal to the kinase domain, deletion of which eliminated the hyperphosphorylation associated with kinase activity. In unstimulated cells, MAP4K4 associates with myosin heavy chain, and the STRIPAK complex maintains the kinase in an inactive state (A). Inhibition of PP2A (e.g. with CalA) results in activation of MAP4K4 (B), with phosphorylation of the activation loop residue (T187) and hyperphosphorylation of the unstructured linker region between the CCD and the C-terminal citron homology (CNH) domain. Strn and Strn4 probably also become phosphorylated. Following treatment with CalA, the association of MAP4K4 with myosin heavy chain is lost.