Summary
Seven overlapping peptides derived from the bovine α1(III)CB4 fragment of collagen
III support static platelet adhesion, and an integrin α2β1-recognition site has been
assigned within this fragment to residues 522-528 of the collagen α1(III) chain; (25).
In this study we found that two of the peptides, CB4(III)-6 and -7, were able to support
platelet adhesion under flow conditions, whereas the other peptides showed either
very little (CB4(III)-1 and -4) or no platelet adhesion at all (CB4(III)-2, -3 and
-5). Using the recombinant leech anti-platelet protein (rLAPP), known to prevent both
α2β1 integrin- and von Willebrand factor (vWF)-binding to collagen, we observed almost
complete inhibition of platelet adhesion to peptides CB4(III)-6 and -7. In solidphase
binding assays rLAPP bound to CB4(III)-6 and -7 and to CB4(III)-6/7, containing the
peptide 6/7 overlap sequence, and not to any other peptide. Our results suggest that
the overlap sequence GPP*-GPRGGAGPP*GPEGGK (single-letter amino acid code, P* = hydroxyproline),
corresponding to residues 523-540 of the α1(III) collagen chain, contains a binding
site for rLAPP. Monoclonal antibodies (MoAbs) directed against the α2 subunit of integrin
α2β1 inhibited platelet adhesion to both CB4(III)-6 and -7 by about 50%, showing that
the α2β1-recognition site in this locality in α1(III)CB4 detected under static conditions
is of sufficient affinity to withstand shear forces. Solid-phase binding studies indicated
that vWF binds to CB4(III)-7 and to a lesser extent to CB4(III)-4. Furthermore, rLAPP
competed with vWF in binding to CB4(III)-7. Our results indicate that residues 541-558
of the α1(III)-chain may contain one of the critical vWF-binding sites involved in
the initial phase of platelet adhesion to collagen III. MoAbs against vWF (A1 and
A3 domain) and glycoprotein (GP)Ib confirmed that vWF is involved in adhesion to CB4(III)-7
and showed that vWF is also involved in adhesion to CB4(III)-6 despite the absence
of direct binding of vWF to the peptide. The existence of α2β1-, vWF- and rLAPP-binding
sites all in close proximity in α1(III)CB4 testifies to the importance of this locus
in collagen III for its platelet reactivity.