The small serine-threonine protein SIP2 interacts with STE12 and is involved in ascospore germination in Sordaria macrospora

In fungi, the homoeodomain protein STE12 controls diverse developmental processes, and derives its regulatory specificity from different protein interactions. We recently showed that in the homothallic ascomycete Sordaria macrospora, STE12 is essential for ascospore development, and is able to interact with the alpha-domain mating-type protein SMTA-1 and the MADS box protein MCM1. To further evaluate the functional roles of STE12, we used the yeast two-hybrid approach to identify new STE12-interacting partners. Using STE12 as bait, a small, serine-threonine-rich protein (designated STE12- interacting protein 2, SIP2) was identified. SIP2 is conserved among members of the fungal class Sordariomycetes. In vivo localization studies revealed that SIP2 was targeted to the nucleus and cytoplasm. The STE12/SIP2 interaction was further confirmed in vivo by bimolecular fluorescence complementation. Nuclear localization of SIP2 was apparently mediated by STE12. Unlike deletion of ste12, deletion of sip2 in S. macrospora led to only a slight decrease in ascospore germination, and no other obvious morphological phenotype. In comparison to the Δste12 single knockout strain, ascospore germination was significantly increased in a Δsip2/ste12 double knockout strain. Our data provide evidence for a regulatory role of the novel fungal protein SIP2 in ascospore germination.