Frazzled regulation of myosin II activity in the Drosophila embryonic CNS

Frazzled (Fra) is a chemoattractive guidance receptor regulating the cytoskeletal dynamics underlying growth cone steering at the Drosophila embryonic midline. Here, by genetically evaluating the role of Rho GTPases in Fra signaling in vivo, we uncover a Rho-dependent pathway apparently regulating conventional myosin II activity. Midline crossing errors induced by expressing activated Cdc42v12 or Racv12 are suppressed by a heterozygous loss of fra4 signaling but, in a Frawt gain-of-function condition, no interaction is detected. In contrast, the frequency of crossovers is enhanced ∼5-fold when Frawt is co-expressed with activated Rhov14 and this interaction specifically requires the cytoplasmic P3 motif of Fra. Expression of Rhov14 and activated MLCK (ctMLCK) synergistically increase ectopic crossovers and both require phosphorylation of the regulatory light chain (Sqh) of myosin II. Abelson tyrosine kinase may also help regulate myosin II activity. Heterozygous abl4 abolishes the midline crossing errors induced by ctMLCK alone or in combination with Frawt; suppression of Rhov14 crossovers is not observed. Interestingly, an interaction between Fra and an activated Abl (Bcr-Abl) also specifically requires the P3 motif. Therefore, the P3 motif of Frazzled appears to initiate Rho and Abl dependent signals to directly or indirectly regulate myosin II activity in growth cones.