SINGLE AMINO-ACID MUTATIONS IN DROSOPHILA FASCIN DISRUPT ACTIN BUNDLING FUNCTION IN-VIVO

Fascins bundle actin filaments into large, tightly packed hexagonal ar rays that support diverse cellular processes including microvillar pro jections and filopodial extensions. In Drosophila, fascin is encoded b y the singed locus. Severe singed mutants have gnarled bristles and ar e female sterile due to a defect in rapid cytoplasm transport during o ogenesis. In this paper, we report the results of a large EMS mutagene sis screen to generate new singed alleles. A mutation that changes gly cine 409 to glutamic acid results in partial inactivation of fascin in vivo, singed(G409E) mutants have kinked bristles and are fertile with a mild nurse cell cytoplasm transport defect. This mutation is in a s mall conserved domain near the C-terminus of fascin. A mutation that c hanges serine 289 to asparagine almost completely inactivates fascin i n vivo; singed(S289N) mutants have gnarled bristles and are sterile du e to a severe defect in nurse cell cytoplasm transport caused by the a bsence of nurse cell cytoplasmic actin bundles. A subsequent EMS mutag enesis screen for dominant suppressors of singed(S289N) sterility reve aled an intragenic suppressor mutation that changes serine 251 to phen ylalanine and restores much of fascin's function. These two mutations, S289N and S251F, draw attention to a central domain in fascin.