The 33-kDa matrix protein SPARC (BM-40, osteonectin) binds several collagen types with moderate affinity. The collagen-binding site resides in helix ␣A of the extracellular calciumbinding domain of SPARC and is partially masked by helix ␣C. Previously, we found that the removal of helix ␣C caused a 10-fold increase in the affinity of SPARC for collagen, and we identified amino acids crucial for binding by site-directed mutagenesis. In this study, we used rotary shadowing, CNBr peptides, and synthetic peptides to map binding sites of SPARC onto collagens I, II, and III. Rotary shadowing and electron microscopy of SPARC-collagen complexes identified a major binding site ϳ180 nm from the C terminus of collagen. SPARC binding was also detected with lower frequency near the matrix metalloproteinase cleavage site. These data fit well with our analysis of SPARC binding to CNBr peptides, denaturation of which abolished binding, indicating triple-helical conformation of collagen to be essential. SPARC binding was substantially decreased in two of seven ␣2(I) mutant procollagen I samples and after N-acetylation of Lys/Hyl side chains in wild-type collagen. Synthetic peptides of collagen III were used to locate the binding sites, and we found SPARC binding activity in a synthetic triple-helical peptide containing the sequence GPOG-PSGPRGQOGVMGFOGPKGNDGAO (where O indicates 4-hydroxyproline), with affinity for SPARC comparable with that of procollagen III. This sequence is conserved among ␣ chains of collagens I, II, III, and V. In vitro collagen fibrillogenesis was delayed in the presence of SPARC, suggesting that SPARC might modulate collagen fibril assembly in vivo.Collagens are major components of the extracellular matrix, comprising ϳ30% of the protein mass of vertebrates. At least 28 collagens have been identified so far (1-3) encoded by 46 separate genes. The defining feature of collagens is a triple-helical domain assembled from three polypeptide strands, called ␣-chains, which each consist of repeating Gly-X-Y sequences, where X is often proline and Y is often hydroxyproline. This primary structure facilitates left-handed poly-proline II helical conformation in each ␣-chain, and three ␣-chains form a righthanded triple helix stabilized by interchain hydrogen bonds (4). To form this structure, Gly is required as every third residue. Its lack of side chains allows contact between ␣-chains, whereas the side chains of residues X and Y are exposed on the surface of the triple helix where they can interact with various molecules. Collagens are major structural proteins, endowing mechanical strength upon the extracellular matrix and also regulating cell behavior through the binding of cell surface receptors and other proteins, such as discoidin domain receptors, a subset of the integrins ␣11, ␣21, ␣101, and ␣111, NG2 proteoglycan, and many matrix molecules (5-7). Nearly 50 different ligands have been shown to interact with collagen, and the binding sites for half of these have been mapped onto collagen I (8). The bind...