IgG Fc-binding protein positively regulates the assembly of pore-forming protein complex βγ-CAT evolved to drive cell vesicular delivery and transport

Cell membranes form barriers for molecule exchange between the cytosol and the extracellular environments. βγ–CAT, a complex of pore-forming protein BmALP1 (two βγ-crystallin domains with an aerolysin pore-forming domain) and the trefoil factor BmTFF3, has been identified in toad Bombina maxima. It plays pivotal roles, via inducing channel formation in various intracellular or extracellular vesicles, as well as in nutrient acquisition, maintaining water balance, and antigen presentation. Thus, such a protein machine should be tightly regulated. Indeed, BmALP3 (a paralog of BmALP1) oxidizes BmALP1 to form a water-soluble polymer, leading to dissociation of the βγ–CAT complex and loss of biological activity. Here, we found that the B. maxima IgG Fc-binding protein (FCGBP), a well-conserved vertebrate mucin-like protein with unknown functions, acted as a positive regulator for βγ–CAT complex assembly. The interactions among FCGBP, BmALP1, and BmTFF3 were revealed by co-immunoprecipitation assays. Interestingly, FCGBP reversed the inhibitory effect of BmALP3 on the βγ–CAT complex. Furthermore, FCGBP reduced BmALP1 polymers and facilitated the assembly of βγ-CAT with the biological pore-forming activity in the presence of BmTFF3. Our findings define the role of FCGBP in mediating the assembly of a pore-forming protein machine evolved to drive cell vesicular delivery and transport.