A protein complex formed by four Ustilago maydis effectors is essential for virulence

Ustilago maydis is a biotrophic fungal pathogen, which causes smut disease in its host plant maize. During colonization U. maydis secretes about 250 effector proteins to suppress plant defense responses and manipulate the host physiology for its own benefit. A majority of these proteins lack functional annotations and their role in virulence remains to be determined.

Effectors were categorized based on their expression pattern during the lifecycle of U. maydis. By focusing on the most highly upregulated genes during the initial interaction, we were able to identify three effectors whose deletion mutants were no longer able to cause disease. Deletion mutants of these three effectors, named stp2, stp3 and stp4 (stop after penetration) were still able to form appressoria and penetrate the plant, but arrested in the epidermal cell layer. The arrest was accompanied by plant defense responses including a disruption of the plant plasma membrane surrounding the fungal hyphae. A similar phenotype was observed for the previously described effectors stp1 and pep1 (Döhlemann et al.). Co-IP/MS experiments using each of these essential effectors revealed that Stp1, Stp3, Stp4 and Pep1 form an effector complex. Interestingly, complex members did not show specific interactions with Stp2 or plant proteins. Our newest findings suggest that not only the presence of the individual complex members, but the formation of the complex itself is necessary for a successful colonization. We will report on our current efforts to identify the cellular compartment where the complex resides, the plant proteins it interacts with and will speculate on the function of the complex and its members.