Abstract
Protein export in eukaryotes can either occur via the classical pathway traversing the endomembrane system or exploit alternative routes summarized as unconventional secretion. Besides multiple examples in higher eukaryotes, unconventional secretion has also been described for fungal proteins with diverse functions in important processes such as development or virulence. Accumulating molecular insights into the different export pathways suggest that unconventional secretion in fungal microorganisms does not follow a common scheme but has evolved multiple times independently. In this study, we review the most prominent examples with a focus on the chitinase Cts1 from the corn smut Ustilago maydis. Cts1 participates in cell separation during budding growth. Recent evidence indicates that the enzyme might be actively translocated into the fragmentation zone connecting dividing mother and daughter cells, where it supports cell division by the degradation of remnant chitin. Importantly, a functional fragmentation zone is prerequisite for Cts1 release. We summarize in detail what is currently known about this potential lock-type mechanism of Cts1 secretion and its connection to the complex regulation of fragmentation zone assembly and cell separation.
Highlights
Protein secretion is of extraordinary importance for living cells
We summarize in detail what is currently known about this potential lock-type mechanism of Cts1 secretion and its connection to the complex regulation of fragmentation zone assembly and cell separation
With respect to the secretory process, both septation factors important for unconventional secretion of Cts1 in U. maydis, Don3 and Don1, are not conserved in S. cerevisiae. This is in line with the fact that the process of septation and cell separation is clearly different from U. maydis and especially that no obvious fragmentation zone exists in S. cerevisiae [72]
Summary
Protein secretion is of extraordinary importance for living cells. The traditional view of protein export in eukaryotes encompasses the signal peptide-mediated transfer through the endomembrane system. After entering the endoplasmic reticulum (ER) and proper folding, proteins are translocated to the Golgi apparatus and packed into secretory vesicles, which fuse with the cytoplasmic membrane to release their content into the extracellular space [1]. On their route, proteins are often subject to pathway-specific post-translational modifications like N-glycosylation or disulfide bond formation [2,3]. Some unconventionally secreted proteins like the fibroblast growth factor 2 (FGF2) have been studied in depth and molecular details of their secretory mechanisms have been uncovered [9]. We summarize recent findings in the field of unconventional secretion for fungal microorganisms with special emphasis on the secretory mechanism of chitinase Cts in the corn smut Ustilago maydis
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