Functional characterization of the Num1 protein

In the corn smut fungus Ustilago maydis, sexual development is initiated by the fusion of two haploid sporidia, resulting in a filamentous growing dikaryon that is capable to infect the plant. Growth of the dikaryon requires an elaborate regulation of cell cycle, migration and distribution of the two nuclei and the polar growth of the hyphae .

We have identified the Num1 protein with a pivotal function during these processes. Num1 is a homologue of human SPF27, one of the core components of the evolutionary conserved Prp19/CDC5 complex, which is an integral component of active spliceosomes and required for intron removal. In addition to regulating spliceosome conformations and fidelity, the complex is known to be involved in the cellular response to DNA damage and cell cycle checkpoint control. 

While vegetative growth of sporidia is not altered in num1 deletion mutants, the hyphae show various polarity defects, delocalized septae and dislocalized nuclei. Using the Yeast Two-Hybrid system, we identified CDC5, another conserved component of the Prp19/CDC5 complex, as Num1 interactor. However, we also identified various proteins with functions during vesicle-mediated transport, in particular the kinesin 1 motor protein. The Num1/Kin1 interaction was verified by Co-immunoprecipitation and Split-YFP analysis. Both num1 and kin1 deletion strains exhibit identical phenotypes with respect to vacuole morphology, filamentous and polar apical growth, corroborating the genetic interaction between Num1 and Kin1.

It is of great interest to investigate whether Num1 has a dual function in long distant transport and splicing, or whether a single function might be a novel connection between these disparate mechanisms.