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Blood, Vol. 94 No. 3 (August 1), 1999:
pp. 976-983
Cyclophilin B Binding to Platelets Supports Calcium-Dependent
Adhesion to Collagen
Fabrice Allain,
Sandrine Durieux,
Agnès Denys,
Mathieu Carpentier, and
Geneviève Spik
From the Laboratoire de Chimie Biologique, Unité Mixte de
Recherche no. 8576 du CNRS, Université des Sciences et
Technologies de Lille, Villeneuve d'Ascq, France.
We have recently reported that cyclophilin B (CyPB), a secreted
cyclosporine-binding protein, could bind to T lymphocytes through
interactions with two types of binding sites. The first ones, referred
to as type I, involve interactions with the conserved domain of CyPB
and promote the endocytosis of surface-bound ligand, while the second
type of binding sites, termed type II, are represented by
glycosaminoglycans (GAG). Here, we further investigated the interactions of CyPB with blood cell populations. In addition to
lymphocytes, CyPB was found to interact mainly with platelets. The
binding is specific, with a dissociation constant (kd) of 9 ± 3 nmol/L and the number of sites estimated at 960 ± 60 per cell. Platelet glycosaminoglycans are not required for the
interactions, but the binding is dramatically reduced by active
cyclosporine derivatives. We then analyzed the biologic effects of CyPB
and found a significant increase in platelet adhesion to collagen. Concurrently, CyPB initiates a transmembranous influx of
Ca2+ and induces the phosphorylation of the P-20 light
chains of myosin. Taken together, the present results demonstrate for
the first time that extracellular CyPB specifically interacts with
platelets through a functional receptor related to the lymphocyte type
I binding sites and might act by regulating the activity of a
receptor-operated membrane Ca2+ channel.

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