The germinal centre kinase Don3 is crucial for unconventional secretion of chitinase Cts1 in Ustilago maydis.

Unconventional secretion has emerged as an increasingly important cellular process in eukaryotic cells. The underlying translocation mechanisms are diverse and often little understood. We study unconventional secretion of chitinase Cts1 in the corn smut fungus Ustilago maydis. This protein participates in the cytokinesis of yeast cells. During budding it localizes to the septated fragmentation zone where it presumably functions in the degradation of remnant chitin to allow separation of mother and daughter cell. However, the mechanistic details of Cts1 export remain unclear. Here we investigated the mechanism of unconventional Cts1 secretion with a focus on cytokinesis. Cell-cycle inhibition experiments supported the hypothesis that Cts1 export is connected to cytokinesis. To substantiate this finding we analysed gene deletion mutants impaired in cell separation and discovered that strains defective in secondary septum formation were affected in Cts1 export. The germinal centre kinase Don3 had a particularly strong influence on unconventional secretion. Using a synthetic switch, we unambiguously verified an essential role of Don3 for cytokinesis-dependent Cts1 export via the fragmentation zone. Thus, we gained novel insights into the mechanism of unconventional secretion and discovered the first regulatory component of this process.

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