A positive regulator of the alternate pathway of complement (PubMed:28264884). It binds to and stabilizes the C3- and C5-convertase enzyme complexes (By similarity). Inhibits CFI-CFH mediated degradation of Inhibits CFI-CFH mediated degradation of Complement C3 beta chain (C3b) (By similarity). In plasma, properdin exists as dimers, trimers or tetramers in the relative proportions of 26:54:20 (By similarity). Interacts with the pro-C3-convertase enzyme complex (C3b-Bb) comprised of Complement C3 beta chain (C3b) and the Complement factor B Bb fragment (Bb), where it binds (via its TSP type-1 5 domain) with C3b and Bb (By similarity). This interaction stabilizes the complex and allows it to become the active C3-convertase enzyme complex (C3b-Bb-FP) (By similarity). Interacts with C3b (PubMed:28264884). Interacts with CFB (By similarity). TSP type-1 domain 0 binds to TSP type-1 domain 4, and TSP type-1 domain 1 binds to TSP type-1 domain 6 (By similarity). These interactions mediate multimerization (By similarity).