View Program - 63rd Annual Drosophila Research Conference

The Drosophila male germline contains both mitotic cells (spermatogonia) and meiotic cells (spermatocytes), and the regulation of cell division in these two cell types is dramatically different. Spermatogonia divide regularly and efficiently; spermatocytes, in contrast, undergo a meiotic G2 prophase that lasts 3.5 days, and the concurrent delay of the meiotic divisions is mediated by fine-tuned control of the temporal expression of core cell cycle components. One such cell cycle factor is Cyclin B (CycB). CycB protein expression is high in mitotic spermatogonia, and then low in immature spermatocytes. CycB protein levels spike again just before spermatocytes enter the meiotic divisions. Published work from our lab has shown that the RNA-binding protein Rbp4 and its co-factor Fest repress cycB translation, mediated by sequences in the 130nt cycB spermatocyte 3’UTR. Subsequent work has revealed that Fest acts as a scaffold protein, binding Lutin (CG1690) and Syp as well as Rbp4. Lut, like Rbp4, is required for repressing cycB translation, although the premature expression of CycB protein in a lut mutant appears to begin later than it does in an rbp4 mutant. We confirmed this using the heat-shock time-course developed in the lab, where bam mutant spermatogonia are subjected to a pulse of wild-type Bam protein under the control of a heat-shock promoter, then differentiate into spermatocytes and later stages in synchrony. Preliminary data indicate that CycB protein is high by 54 hours post-heat-shock (PHS) in rbp4, by 92h PHS in lut, and by 104h (but not 100h) PHS in wild-type. This result raises the possibility that Rbp4 and Lut, while both belonging to the same complex, could be repressing cycB translation via two distinct mechanisms, perhaps at sequential steps. In contrast, we have found that Syp is required for CycB accumulation in mature spermatocytes. Syp, like Rbp4, binds the 130nt cycB spermatocyte 3’UTR. Curiously, Syp binds to Fest and can co-precipitate with both Rbp4 and Lut in the presence but not absence of Fest. Spermatocytes in the rbp4 syp double mutant show a transient, early expression of CycB, whereas CycB is never detectable in lut syp spermatocytes, suggesting that Syp may act by countering Rbp4 function (but not Lut function). The dynamic off-to-on effect on cycB translation is not dictated by any changes in the core interactions within the complex between 72h PHS and 104h PHS.

Regulation of cycB translation by a four-protein complex in Drosophila spermatocytes

**Regulation of cycB translation by a four-protein complex in Drosophila spermatocytes**