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Structural and functional analysis of the cerato-platanin-like effector protein Cpl1 suggests diverging functions in smut fungi

Plant pathogenic fungi are causative agents of the majority of plant diseases and can lead to severe crop loss in infected populations. Fungal colonization is achieved by combining different strategies, such as avoiding and counteracting the plant immune system and manipulating the host metabolome. Of major importance are effector proteins secreted by the fungi that fulfill diverse functions to support the infection process. Most of these proteins are highly specialized and structural and biochemical information is often absent. Here, we present the atomic structures of the cerato-platanin-like protein Cpl1 from Ustilago maydis and its homolog Uvi2 from Ustilago hordei. Both proteins adopt a double-Ψ-β-barrel architecture reminiscent of cerato-platanin proteins, a class so far not described in smut fungi. Our structure-function analysis shows that Cpl1 binds to soluble chitin fragments via two extended grooves at the dimer interface of the two monomer molecules. This carbohydrate-binding mode has not been observed previously and expands the repertoire of chitin-binding proteins. Cpl1 localizes to the cell wall of U. maydis and specifically enriches cell-wall degrading and -decorating proteins during maize infection. The architecture of Cpl1 harboring four surface exposed loop regions supports the idea that it might play a role in spatial coordination of these proteins. While deletion of cpl1 has only mild effects on the virulence of U. maydis, a recent study showed that deletion of uvi2 strongly impairs U. hordei virulence. Our structural comparison between Cpl1 and Uvi2 reveals sequence variations in the loop regions which might explain a diverging function.