The Predicted Structure of S. cerevisiae Ssp1 Reveals Parallel Evolution in the Pil1 BAR Domain Family Proteins of Ascomycetes

BAR domains are a superfamily of widely conserved membrane binding motifs. In fungi, Pil1 family proteins are BAR domain containing proteins involved in organizing the plasma membrane. <i>S. pombe</i> encodes a sporulation-specific Pil1 family protein, Meu14, which has a specialized role in shaping the forespore membrane during sporulation. The functional analog of Meu14 in <i>S. cerevisiae</i> is Ssp1. While Ssp1 has no primary sequence homology to Pil1 or Meu14, AlphaFold predicts that it contains a Pil1-related BAR domain. Consistent with this structural prediction, mutation of residues in the putative lipid binding face of Ssp1 or in a residue implicated in multimerization disrupt sporulation. Characterization of the mutant proteins indicates that the BAR domain is necessary for recruitment of Ssp1 to the highly curved leading edge of the prospore membrane and multimerization of Ssp1 at that location is required for assembly of the leading edge complex. The distribution of Pil1 family proteins across an evolutionary tree of <i>Ascomycetes</i> reveals that Meu14 and Ssp1 arose independently in the lineages leading to <i>S. pombe</i> and <i>S. cerevisiae</i>, respectively.