pore-forming module of Physarum polycephalum spherulin-2a, Plodia interpunctella follicular epithelium yolk protein subunit YP4, and similar aerolysin-type beta-barrel pore-forming proteins
Spherulin 2a is a coat glycoprotein produced during encystment from the slime mold, Physarum polycephalum. YP4, is one of two subunits of the follicular epithelium yolk protein from Plodia interpunctella and other pyralid moths; it is produced in the follicle cells during vitellogenesis, and after secretion it is taken up into the oocyte and stored in the yolk spheres for utilization during embryogenesis. Members of this group belong to the aerolysin family of beta-pore-forming proteins (beta-PFPs). PFPs are generally secreted as water-soluble monomers, which upon binding to target lipid membranes, oligomerize and form transmembrane pores harmful to cells. Beta-PFPs form pores by transmembrane beta-barrels. Aerolysin-type beta-PFPs are believed to use an amphipathic beta-hairpin to form the beta-barrel, are found in all kingdoms of life and many are bacterial toxins. In addition to having a role in microbial infection, they have potential as biotechnological sensors and delivery systems. They share a similar monomeric architecture, with a variable membrane-binding domain and a structurally conserved pore-forming region. A significant portion of the monomeric subunit structure is re-organized to form the pore. Oligomers formed by members of the aerolysin family include: hepta- (aerolysin), octa- (Dln1), and nonameric oligomers (lysenin and monalysin).
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