Metabolism of activated complement component C3 is mediated by the low density lipoprotein receptor-related protein/alpha(2)-macroglobulin receptor

Complement component 3 (C3) and alpha(2)-macroglobulin evolved from a commo n, evolutionarily old, ancestor gene, Low density lipoprotein-receptor-rela ted protein/alpha(2)-macroglobulin receptor (LRP/alpha(2)MR), a member of t he low density lipoprotein receptor family, is responsible for the clearanc e of alpha(2)-macroglobulin-protease complexes. In this study, we examined whether C3 has conserved affinity for LRP/alpha(2)MR. Ligand blot experimen ts with human I-125-C3 On endosomal proteins show binding to a 600-kDa prot ein, indistinguishable from LRP/alpha(2)MR by the following criteria: it is competed by receptor-associated protein (the 39-kDa receptor-associated pr otein that impairs binding of all ligands to LRP/alpha 2MR) and by lactofer rin and Pseudomonas exotoxin, other well known ligands of the multifunction al receptor. Binding of C3 is sensitive to reduction of the receptor and is Ca2+-dependent. All these features are typical for cysteine-rich binding r epeats of the low density lipoprotein receptor family. In LRP/alpha(2)MR, t hey are found in four cassettes (2, 8, 10, and 11 repeats), Ligand blotting to chicken LR8 demonstrates that a single 8-fold repeat is sufficient for binding. Confocal microscopy visualizes initial surface labeling of human f ibroblasts incubated with fluorescent labeled C3, which changes after 5 min to an intracellular vesicular staining pattern that is abolished in the pr esence of receptor-associated protein, Cell uptake is abolished in mouse fi broblasts deficient in LRP/alpha(2)MR. Native plasma C3 is not internalized . We demonstrate that the capacity to internalize C3 is saturable and exhib its a K-D value of 17 nM. After intravenous injection, rat hepatocytes accu mulate C3 in sedimentable vesicles with a density typical for endosomes. In conclusion, our ligand blot and uptake studies demonstrate the competence of the LRP/alpha(2)MR to bind arid endocytose C3 and provide evidence for a n LRP/alpha(2)MR-mediated system participating in C3 metabolism.