AAK1 is a serine/threonine kinase that is thoμght to coordinate the recruitment of AP-2 to receptors containing tyrosine-based internalization motifs by phosphorylating the µ2 subunit. There is strong evidence that this protein is the endogenous mu 2 kinase and plays a regulatory role in clathrin-mediated endocytosis, including the regulation of fatty acid synthesis and cholesterol synthesis.