Tip growth is closely tied to fungal pathogenicity. Budding yeast Spa2 (the homolog of GIT1 and GIT2 in mammals), a multi-domain protein and member of the polarisome, orchestrates tip growth in yeasts and other fungi. We identified a conserved short linear motif in the Rab GTPase-activating proteins (RabGAPs) Msb3 and Msb4, and the MAP kinase kinases Ste7 and Mkk1, which mediates their interaction with Spa2. AlphaFold predictions suggest that these initially unstructured motifs adopt an α-helical conformation upon binding to the hydrophobic cleft in the N-terminal domain of Spa2. Altering the predicted key contact residues in either Spa2 or the motif reduces complex stability. Such mutations also cause mis-localization of Msb3, Msb4 and Ste7 within the cell. Deleting the motif in Msb3 or Msb4 abolishes tip-directed growth of the yeast bud. Protein assemblies that spatially confine secretion to specific membrane regions are a common feature of eukaryotic cells. Accordingly, complexes between proteins with this motif and Spa2 were predicted in orthologs and paralogs across selected Opisthokonta, including pathogenic fungi and humans. A search for functional motifs in conformationally flexible regions of all yeast proteins identified Dse3 as a novel Spa2-binding partner.

• PMID: 41527849
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Journal Article

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Bareis L, Siewert A, Grupp B, Bergner T, Read C, Timmermann S, Schmid N, Johnsson N

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