Recognition at cell surfaces: phytohaemagglutinin-lymphocyte interaction.

Many aspects of cell behaviour are regulated by the interaction of extracellular ligands with specific receptors exposed on the cell surface. The receptors correspond to membrane proteins and expecially glycoproteins. A key event in regulation is the transmission across the surface membrane of the information resulting from receptor-ligand interaction. The activation of lymphocytes by Phaseolus vulgaris phytohaemagglutinin (PHA) provides a convenient experimental model for the study of the molecular basis of receptor-ligand interaction and the molecular consequences of interaction. The receptor mediating lymphocyte activation by PHA is probably a unique glycoprotein which is present to the extent of about 3 X 10(4) molecules/cell. The PHA-receptor complex solubilized in 1% sodium deoxycholate has a molecular size of about 3 X 10(5). The primary event in the activation process is probably an increase in the permeability of the surface membrane to Ca2+. This may be achieved by PHA cross-linking ('patching') the receptors to form a polar channel that permits an influx of Ca2+.