C3bi receptor (complement receptor type 3) recognizes a region of complement protein C3 containing the sequence Arg-Gly-Asp

New perspectives in cell adhesion: RGD and integrins

Rapid progress has been made in the understanding of the molecular interactions that result in cell adhesion. Many adhesive proteins present in extracellular matrices and in the blood contain the tripeptide arginine-glycine-aspartic acid (RGD) as their cell recognition site. These proteins include fibronectin, vitronectin, osteopontin, collagens, thrombospondin, fibrinogen, and von Willebrand factor. The RGD sequences of each of the adhesive proteins are recognized by at least one member of a family of structurally related receptors, integrins, which are heterodimeric proteins with two membrane-spanning subunits. Some of these receptors bind to the RGD sequence of a single adhesion protein only, whereas others recognize groups of them. The conformation of the RGD sequence in the individual proteins may be critical to this recognition specificity. On the cytoplasmic side of the plasma membrane, the receptors connect the extracellular matrix to the cytoskeleton. More than ten proved or suspected RGD-containing adhesion-promoting proteins have already been identified, and the integrin family includes at least as many receptors recognizing these proteins. Together, the adhesion proteins and their receptors constitute a versatile recognition system providing cells with anchorage, traction for migration, and signals for polarity, position, differentiation, and possibly growth.

Aggregation of complement receptors on human neutrophils in the absence of ligand

C3bi receptors (CR3) on human polymorphonuclear leukocytes (PMN) bind ligand-coated particles and promote their ingestion. The binding activity of CR3 is not constitutive but is transiently enabled by phorbol esters (Wright, S. D., and B. D. Meyer, 1986, J. Immunol. 136:1759-1764). Our observations indicate that the capacity of CR3 to bind ligand is tightly correlated with the degree of ligand-independent aggregation of the receptor in the plane of the membrane. Fixed PMN were labeled with anti-CR3 monoclonal antibodies and streptavidin colloidal gold before viewing in the electron microscope either en face or in thin section. On unstimulated PMN, gold particles marking CR3 were dispersed randomly. Stimulation of PMN for 25 min with phorbol myristate acetate (PMA) dramatically enhances binding of C3bi-coated particles, and the CR3 on such stimulated cells was observed in clusters containing more than six gold particles. CR3 was not aggregated over coated pits. After 50 min in PMA, the binding activity of CR3 falls, and the distribution of CR3 was again observed to be disperse. If a hydrophilic phorbol ester was washed away after a 20-min stimulation, binding activity remains elevated for at least 50 min, and CR3 remained aggregated. Thus, clustering of CR3 was temporally correlated with its ability to bind ligand and initiate phagocytosis. Unlike CR3, Fc receptors and HLA did not exhibit changes in their aggregation state in response to PMA. Treating PMN with formyl-methionyl-leucyl-phenylalanine, which enhances expression of CR3 but not its function, did not lead to aggregation of CR3. These observations suggest that a clustered configuration is a precondition necessary for binding ligand and signaling phagocytosis.

The very late antigen family of heterodimers is part of a superfamily of molecules involved in adhesion and embryogenesis

The very late antigen (VLA) protein family contains at least five related heterodimers, including a fibronectin receptor structure, and probably other cell substrate adhesion receptors. These cell-surface VLA proteins were immunopurified from human placenta (VLA-1, VLA-3, and VLA-5), platelets (VLA-2), and Molt-4 cells (VLA-4) using a series of monoclonal antibody-Sepharose immunoaffinity columns. After further purification by gel electrophoresis, the N-terminal amino acid sequence for each of the five VLA alpha subunits was determined. In the first 14 positions, the five VLA alpha subunits showed an average of 42% homology to each other, rising to 59% including conservative amino acid substitutions. In addition, the alpha subunits from the LFA-1, Mac-1 (CR-3), and p150,95 family of heterodimers, the vitronectin receptor-platelet GPIIb/IIIa family, and a position-specific (PS) antigen important in Drosophila embryogenesis each showed average homologies of 31-40% to individual VLA alpha sequences and 46-52% homology to VLA alpha subunits including conservative substitutions. Taken together, these results suggest that the VLA proteins, the LFA-1, Mac-1, and p150,95 family, the GPIIb/IIIa, vitronectin receptor family, and the Drosophila PS antigens have evolved as four subgroups in a highly conserved supergene family of receptors involved in fundamentally important functions, such as cell adhesion, migration, and embryogenesis.