Abstract
Protein kinases are frequently regulated by intramolecular autoinhibitory interactions between protein modules that are reversed when these modules bind other 'activating' protein or membrane-bound targets. One group of kinases, the MAP/microtubule affinity-regulating kinases (MARKs) contain a poorly understood regulatory module, the KA1 (kinase associated-1) domain, at their C-terminus. KA1 domains from MARK1 and several related kinases from yeast to humans have been shown to bind membranes containing anionic phospholipids, and peptide ligands have also been reported. Deleting or mutating the C-terminal KA1 domain has been reported to activate the kinase in which it is found - also suggesting an intramolecular autoinhibitory role. Here, we show that the KA1 domain of human MARK1 interacts with, and inhibits, the MARK1 kinase domain. Using site-directed mutagenesis, we identify residues in the KA1 domain required for this autoinhibitory activity, and find that residues involved in autoinhibition and in anionic phospholipid binding are the same. We also demonstrate that a 'mini' MARK1 becomes activated upon association with vesicles containing anionic phospholipids, but only if the protein is targeted to these vesicles by a second signal. These studies provide a mechanistic basis for understanding how MARK1 and its relatives may require more than one signal at the membrane surface to control their activation at the correct location and time. MARK family kinases have been implicated in a plethora of disease states including Alzheimer's, cancer, and autism, so advancing our understanding of their regulatory mechanisms may ultimately have therapeutic value.