We show that Tramtrack (TTK88) expression represses neuronal fate determination in the developing Drosophila eye. Phyllopod (PHYL) acts to antagonize this repression by a mechanism that requires Seven In Absentia (SINA) and is associated with decreased TTK88 protein levels, but not reduced ttk88 gene transcription or mRNA stability. We present evidence that SINA, PHYL, and TTK88 physically interact and that SINA interacts genetically and physically with UBCD1, a component of the ubiquitin-dependent protein degradation pathway. Our results suggest a model in which activation of the Sevenless receptor tyrosine kinase induces PHYL expression, which then acts with SINA to target the transcriptional repressor TTK88 for degradation, thereby promoting R7 cell fate specification.
Bibliographical noteFunding Information:
Correspondence should be addressed to G. M. R. We thank B. Dickson for fly stocks, S. Harrison for the pRmHa3-TTK88-Myc construct, E. O'Neill for the GST-PNT P2 construct, and D. Read for α-TTK88 antibodies. R. Brent and R. Finley generously provided many two-hybrid constructs and yeast strains used in the two-hybrid experiments, and I. Rebay and M. Therrien provided several unpublished two-hybrid constructs. The authors thank Duncan Stuart for assistance with confocal microscopy, M. Jeans and B. Ho for assistance with P element–mediated transformation, C. Suh and N. Solomon for assistance with DNA sequencing, P. Sicurello for assistance with scanning electron microscopy, and P. Schow for assistance with cell flow cytometry. We are grateful to R. Tjian, M. Levine, M. Therrien, I. Rebay, B. A. Hay, and S. D. Harrison for critical reading of this manuscript; to D. K. Morrison and S. Hansen for helpful advice on the protein–protein interaction assays; and to our colleagues in the Rubin lab for helpful discussions throughout the course of this work. A. H. T is supported by a National Institutes of Health postdoctoral fellowship. G. M. R. is a Howard Hughes Medical Institute Investigator.