The actin-regulating kinase Prk1p negatively regulates Scd5p, a suppressor of clathrin deficiency, in actin organization and endocytosis

Endocytosis is a dynamic process requiring a network of interacting proteins that assemble and disassemble during cargo capture and vesicle formation. A major mechanism for regulation of this process involves the reversible phosphorylation of endocytic factors [1–3]. Recently, members of a new kinase family, the Ark/Prk kinases, which include mammalian AAK1 and GAK as well as yeast Prk1p, Ark1p, and Akl1p, were shown to regulate components of the endocytic machinery [4]. These include animal AP-1/AP-2 μ chains and yeast Pan1p (Eps15-like), Sla1p, and epsins [2, 5–10], but other potential targets are likely. SCD5, an essential yeast gene, was identified as a suppressor of clathrin deficiency [11, 12]. We also showed that Scd5p is required for normal cortical actin organization and endocytosis, possibly as a targeting subunit for protein phosphatase type 1 (PP1) [13, 14]. Scd5p contains a central triple repeat (3R) motif related to a known Prk1p consensus phosphorylation site L/IxxQxTG [8, 10], except that Q is replaced by T. In this study we demonstrate that the Scd5p 3R sequence is phosphorylated by Prk1p to negatively regulate Scd5p. Furthermore, we show that Prk1p, Ark1p, and Akl1p have different substrate specificities and play distinct roles in actin organization and endocytosis.