Drosophila LKB1 is required for the assembly of the polarized actin structure that allows spermatid individualization

In mammals, a testis-specific isoform of the protein kinase LKB1 is required for spermiogenesis, but its exact function and specificity are not known. Human LKB1 rescues the functions of Drosophila Lkb1 essential for viability, but these males are sterile, revealing a new function for this genes in fly. We also identified a testis-specific transcript generated by an alternative promoter and that only differs by a longer 5'UTR. We show that dLKB1 is required in the germline for the formation of the actin cone, the polarized structure that allows spermatid individualization and cytoplasm excess extrusion during spermiogenesis. Three of the nine LKB1 classical targets in the Drosophila genome (AMPK, NUAK and KP78b) are required for proper spermiogenesis, but later than dLKB1. dLkb1 mutant phenotype is reminiscent of that of myosin V mutants, and both proteins show a dynamic localization profile before actin cone formation. Together, these data highlight a new dLKB1 function and suggest that dLKB1 posttranscriptional regulation in testis and involvement in spermatid morphogenesis are evolutionarily conserved features.