Complement is an essential component of innate immunity. Its activation results in the assembly of unstable protease complexes, denominated C3/C5 convertases, leading to inflammation and lysis. Regulatory proteins inactivate C3/C5 convertases on host surfaces to avoid collateral tissue damage. On pathogen surfaces, properdin stabilizes C3/C5 convertases to efficiently fight infection. How properdin performs this function is, however, unclear. Using electron microscopy we show that the N-and C-terminal ends of adjacent monomers in properdin oligomers conform a curly vertex that holds together the AP convertase, interacting with both the C345C and vWA domains of C3b and Bb, respectively. Properdin also promotes a large displacement of the TED (thioestercontaining domain) and CUB (complement protein subcomponents C1r/C1s, urchin embryonic growth factor and bone morphogenetic protein 1) domains of C3b, which likely impairs C3-convertase inactivation by regulatory proteins. The combined effect of molecular cross-linking and structural reorganization increases stability of the C3 convertase and facilitates recruitment of fluid-phase C3 convertase to the cell surfaces. Our model explains how properdin mediates the assembly of stabilized C3/C5-convertase clusters, which helps to localize complement amplification to pathogen surfaces. ; This work was funded by the Autonomous Region of Madrid (S2010/BMD-2316 to S.R.d.C. and O.L.), the Ramón Areces Foundation (O.L.), and the Spanish government (SAF2011-22988 to O.L. and SAF2011-26583 to S.R.d.C.). O.L. is additionally supported by Red Temática de Investigación Cooperativa en Cáncer (RD06/0020/1001), and S.R.d.C. is also supported by the Fundación Renal Iñigo Alvarez de Toledo and the Seventh Framework Programme European Union Project EURenOmics (European Consortium for High-Throughput Research in Rare Kidney Diseases-305608).