The fungal protein Jps1 facilitates unconventional protein secretion through a direct phosphoinositide interaction

Protein secretion is indispensable for essential cellular processes in eukaryotic cells, contributing significantly to nutrient acquisition, defense or communication. Alternative pathways bypassing the endomembrane system collectively referred to as unconventional secretion are gaining increasing attention. A number of important molecules such as cytokines, fibroblast growth factor or viral proteins are being exported through these mechanistically diverse pathways. In the fungal model Ustilago maydis , cytokinesis-dependent unconventional secretion mediates export of the chitinase Cts1 via the fragmentation zone. This membrane-rich compartment is formed during cytokinesis between mother and daughter cells. Recently, we identified Jps1, a previously uncharacterized protein, as a crucial factor for Cts1 localization and export. Combining biochemical experiments and in vivo studies, we here uncover two pivotal features of Jps1: phosphoinositide (PIP) binding and dimerization. Our findings reveal that Jps1 does not harbor a canonical PIP-binding domain but instead specifically binds to PI(4,5)P2, likely via basic residues. A conserved structural core domain of Jps1 mediates dimerization, while the flexible variable regions suggest potential diversification in different basidiomycetous species. The accumulation of Jps1 in the fragmentation zone, facilitated by its PIP affinity, provides a direct membrane recognition mechanism. This discovery sheds light on a previously unknown key feature of Jps1, elucidating its role in supporting Cts1 secretion, and representing a crucial step towards understanding the broader implications of unconventional secretion in eukaryotic cells.