Figure 12.

A model for SpTransformer (SpTrf) protein functions for clearance of bacteria from the coelomic fluid (CF). Individual phagocytes secrete a single SpTrf protein variant (41), which is illustrated by individual phagocytes (red, green, blue) producing different (color coded) SpTrf protein variants. Bioinformatic predictions of many deduced SpTrf sequences and circular dichroism results for rSpTransformer-E1 (4) indicate that these proteins are likely intrinsically disordered proteins (IDPs) (squiggles). Upon interaction with or binding to targets in the CF, they transform to α helical structures (corkscrews). Whether the SpTrf proteins associate directly with phospholipids on the surface of small phagocytes (green cell) or whether SpTrf proteins associate with any phagocyte type through putative membrane receptor(s) (black rectangles) remain unknown and await investigation. When vesicle membranes fuse with the cell membrane, the membrane-bound SpTrf proteins are exposed on the surface of the small phagocyte (green cell) (44). Other SpTrf proteins that are secreted by nearby polygonal phagocytes and released into the CF likely bind quickly to pathogens through pathogen-associated molecular patterns (lipopolysaccharide, flagellin, or both) on the pathogen surface and swiftly transform from IDPs to proteins with ordered structure forming helices. Alternatively, secreted SpTrf proteins may bind to the surface of small phagocytes through multimerization with other membrane-bound SpTrf proteins, or may bind directly to phospholipids or to putative receptor(s) (black rectangles). The secreted SpTrf proteins that bind to pathogens may function as opsonins and trigger phagocytosis and pathogen clearance. The insert at the top left illustrates a theoretical clustering of phosphatidic acid (green triangles) in the outer leaflet of a phagocyte plasma membrane (represented as the double black line) by SpTrf proteins bound to the bacterium and induce the concave curvature in the membrane that may aid in the formation of the phagosome and uptake of a microbe. Other mechanisms that are known to be involved with phagosome formation are not shown.