Unlike these previous studies, we extended the confirmation of incompatibility activity to a functional analysis of the un-24 nonself recognition system, initiating an understanding of its mechanisms.

Interestingly, and unlike the filamentous fungi, a vegetative incompatibility MAPK inhibitor system has not been described in yeast and in silico experiments showed that yeast lacks homologs to several heterokaryon incompatibility domains found in filamentous ascomycete fungi [12]. Nevertheless, our work shows that such a system can operate in yeast. As demonstrated here, heterologous expression of nonself recognition factors in yeast can also lead to check details fundamental insights into mechanisms of activity and control of nonself recognition factors. In such a system, core interactions of incompatibility domains can be studied without a confounding effect of other potentially interacting

incompatibility factors. In the future, it would be interesting to determine if these incompatibility reactions can be enhanced in the yeast system by the addition of other known incompatibility factors from N. crassa. For example, it is known that the allelic un-24 incompatibility in N. crassa is significantly strengthened by non-allelic interactions with het-6 factors [15]. One emerging trend observed with heterokaryon incompatibility systems is the involvement of protein-protein interactions that trigger cell death. This is particularly evident in the het-c system of N. crassa[35] and the het-s system in P. anserina[24].

Thiamine-diphosphate kinase Our results indicate that un-24-associated incompatibility is likewise mediated by protein interactions. When expressed at low levels, the PAp domain apparently forms a complex with Rnr1p that results in incompatibility-like phenotypes in yeast. The observed “toxicity” of the Rnr1p-PAp complex in yeast is consistent with incompatibility associated with coexpression of PA and OR alleles of un-24 in N. crassa[15] and with a recently published study that demonstrates that the C-terminus of un-24 PA is able to form a non-reducible complex with UN-24OR in N. crassa, the presence of which is correlated with incompatibility [36]. Unlike N. crassa where there is a single gene (un-24) encoding the RNR large subunit, yeast contains the paralogs RNR1 and RNR3; RNR1 is cell-cycle regulated and used under normal cellular growth, and RNR3 is upregulated in BIBW2992 mw response to DNA damage [37]. The partial redundancy of Rnr1p and Rnr3p may alleviate some toxic effects of expressing PAp in yeast.