The sterol carrier protein 2 in Ustilago maydis has a virulence function

Based on detecting peptides of the peroxisomal sterol carrier protein 2 (Scp2) of U. maydis in apoplastic fluid, we considered the possibility that Scp2 might function as an unconventially secreted effector protein. We used reverse genetics approaches to delete the scp2 gene and showed that scp2 mutants were strongly attenuated in virulence and this defect manifested itself already during cuticle penetration. scp2 mutants were neither affected in growth nor showed defects in peroxisomal β-oxidation. While we could detect a small amount of secreted Scp2 in culture supernatants, we failed to demonstrate that this is in any way related to virulence. Consistently, we could not rescue the virulence defect of scp2 mutants by expressing Scp2 via the conventional secretion route. A biologically active GFP-Scp2 protein localized to peroxisomes 

U. maydis scp2 localizes to peroxisomes. Shown are budding U. maydis cells (DIC, left panel) expressing a peroxisomal marker protein fused to mCherry (middle) and co-expressing GFP-Scp2 (right). The marker protein and GFP-Scp2 co-localize.

and peroxisomal targeting was necessary for its virulence function. scp2 mutants displayed an altered localization of peroxisomes. Our results make it likely that the virulence function for Scp2 during penetration is carried out by Scp2 in peroxisomes. We speculate Scp2 affects the lipid composition of membranes and in this way ensures the even cellular distribution of peroxisomes.

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