Ack1 regulates a macromolecular complex involved in nucleotide synthesis. Todd Strochlic, Alana O'Reilly, Jeffrey Peterson. Cancer Biology Program, Fox Chase Cancer Center, Philadelphia, PA.

   Ack1 (activated cdc42-associated kinase 1) is a poorly characterized non-receptor tyrosine kinase implicated in tumor growth and metastasis in humans. To gain insight into the biological functions of Ack1, we analyzed the role of the homologous protein, DAck, in Drosophila melanogaster. DACK-deficient female flies display reduced fertility and defects in oogenesis characterized by disruption of plasma membranes between germline cells. These phenotypes can be rescued by transgenic expression of wild-type DAck but not a kinase-dead mutant, indicating that DAck kinase activity is critical for oogenesis in the fly. In Drosophila egg chambers, DAck localizes to unusual cytoplasmic filaments that also contain two metabolic enzymes: cytidine triphosphate synthase (CTPS) and inosine- 5-monophosphate dehydrogenase (IMPDH). These enzymes catalyze the rate-limiting steps in the biosynthesis of CTP and GTP, respectively, and consequently we have named this macromolecular assembly of enzymes FINS (filaments involved in nucleotide synthesis). In addition to its role as an essential nucleotide, CTP is required to generate CDP-linked intermediates in the synthesis of membrane phospholipids. Female flies with reduced levels of CTPS are sterile and exhibit membrane defects that phenocopy those observed in DACK-deficient flies, suggesting that DAck may modulate the function of the FINS complex. Indeed, FINS in DACK-deficient flies are smaller and fragmented, indicating a role for DAck in maintaining the functional integrity of this complex. Importantly, the FINS complex is evolutionarily conserved. We have detected autophosphorylated (activated) Ack1 as a component of FINS in mammalian cells grown in nucleotide-depleted conditions, suggesting that FINS assemble in response to increased nucleotide demand and that CTPS and IMPDH are likely active in these structures. Taken together, these results implicate Ack1 in the regulation and coordination of nucleotide metabolism in both flies and mammals.