Chigno/CG11180 and SUMO are Chinmo-interacting proteins with a role in Drosophila testes somatic support cells

Stem cells are critical for replenishment of cells lost to death, damage or differentiation. Drosophila testes are a key model system for elucidating mechanisms regulating stem cell maintenance and differentiation. An intriguing gene identified through such studies is the transcription factor, chronologically inappropriate morphogenesis (Chinmo). Chinmo is a downstream effector of the Jak-STAT signaling pathway that acts in testis somatic stem cells to ensure maintenance of male stem cell fate and sexual identity. Defects in these processes can lead to infertility and the formation of germ cell tumors. While Chinmo's effect on testis stem cell behavior has been investigated in detail, there is still much to be learned about its structure, function, and interactions with other proteins. Using a two-hybrid screen, we find that Chinmo interacts with itself, the small ubiquitin-like modifier SUMO, the novel protein CG11180, and four other proteins (CG4318, Ova (ovaries absent), Taf3 (TBP-associated factor 3), and CG18269). Since both Chinmo and CG11180 contain sumoylation sites and SUMO-interacting motifs (SIMs), we analyzed their interaction in more detail. Using site-directed mutagenesis of a unique SIM in CG11180, we demonstrate that Chinmo's interaction with CG11180 is SUMO-dependent. Furthermore, to assess the functional relevance of both SUMO and CG11180, we performed RNAi-mediated knockdown of both proteins in somatic cells of the Drosophila testis. Using this approach, we find that CG11180 and SUMO are required in somatic cells of adult testes, and that reduction of either protein causes formation of germ cell tumors. Overall, our work suggests that SUMO may be involved in the interaction of Chinmo and CG11180 and that these genes are required in somatic cells of the adult Drosophila testis. Consistent with the CG11180 knockdown phenotype in male testes, and to underscore its connection to Chinmo, we propose the name Chigno (Childless Gambino) for CG11180.