THE ROLE IN CELL-BINDING OF A BETA-BEND WITHIN THE TRIPLE-HELICAL REGION IN COLLAGEN ALPHA-1(I) CHAIN - STRUCTURAL AND BIOLOGICAL EVIDENCE FOR CONFORMATIONAL TAUTOMERISM ON FIBER SURFACE

In its physiological solid state, type I collagen serves as a host for many types of cells. Only the molecules on fiber surface are availabl e for interaction. In this interfacial environment, the conformation o f a cell binding domain can be expected to fluctuate between the colla gen fold and a distinctive non-collagen molecular marker for recogniti on and allosteric binding. If the cell binding domain can be localized in contiguous residues within the exposed half of a turn of the tripl e helix (approximately 15 residues), the need for extensive structural modification and unraveling of the triple helix is avoided. We examin ed the conformational preferences and biological activity of a synthet ic 15-residue peptide (P-15), analogous to the sequence (766)GTPGPQGIA GQRGVV(780) in the alpha 1(I) chain. Theoretical studies showed a high potential for a stable beta-bend for the central GIAG sequence. The f lanking sequences showed facile transition to extended conformations. Circular dichroism of the synthetic peptide in anisotropic solvents co nfirmed the presence of beta-strand and beta-bend structures. P-15 inh ibited fibroblast binding to collagen in a concentration dependent man ner, with near maximal inhibition occurring at a concentration of 7.2x 10(-6) M. The temporal pattern of cell attachment was altered markedly in the presence of P-15. No inhibition was seen with a peptide P-15(A I), an analogue of P-15 with the central IA residues reversed to AI or with collagen-related peptides (Pro-Pro-Gly)(10), (Pro-Hyp-Gly)(10), and polyproline, and with several unrelated peptides. Our studies sugg est a molecular mechanism for cell binding to collagen fibers based on a conformational transition in collagen molecules on the fiber surfac e. Since the energy barrier between the collagen fold and beta-strand conformation is low, a local conformational change may be possible in molecules on the fiber surface because of their location in an anisotr opic environment. Our observations also suggest that the sequence inco rporated in P-15 may be a specific ligand for cells. Unlike other repo rted cell binding peptides, the residues involved in this interaction are non-polar.