Genetic analysis of viable Hsp90 alleles reveals a critical role in Drosophila spermatogenesis

The Hsp90 chaperone protein maintains the activities of a remarkable variet y of signal transducers, but its most critical functions in the context of the whole organism are unknown. Point mutations of Hsp83 (the Drosophila Hs p90 gene) obtained in two different screens are lethal as homozygotes. We r eport that eight transheterozygous mutant combinations produce viable adult s. All exhibit the same developmental defects: sterile males and sterile or weakly fertile females. We also report that scratch, a previously identifi ed male-sterile mutation, is an allele of Hsp82 with a P-element insertion in the intron that reduces expression. Thus, it is a simple reduction in Hs p90 function, rather than possible altered functions in the point mutants, that leads to male sterility. As shown by light and electron microscopy, al l stages of spermatogenesis involving micro tubule function are affected, f rom early mitotic divisions to later stages of sperm maturation, individual ization, and motility. Aberrant microtubules are prominent in yeast cells c arrying mutations in HSP82 (the yeast Hsp90 gene), confirming that Hsp90 fu nction is connected to microtubule dynamics and that this connection is hig hly conserved. A small fraction of Hsp90 copurifies with taxol-stabilized m icrotubule proteins in Drosophila embryo extracts, but Hsp90 does not remai n associated with microtubules through repeated temperature-induced assembl y and disassembly reactions. If the spermatogenesis phenotypes are due to d efects in microtubule dynamics, we suggest these are indirect, reflecting a role fur Hsp90 in maintaining critical signal transduction I,always and mi crotubule effecters, rather than a direct role in the assembly and disassem bly of microtubules themselves.