Phytohemagglutinin mitogenic proteins. Structural evidence for a family of isomitogenic proteins.

The phytohemagglutinin (PHAP) glycoproteins derived from Phaseolus vulgaris consist of five isomitogens that are tetrameric structures made up of two different glycoprotein subunits. Although identical in size (mol wt = 34,000), the subunits differ in their isoelectric points and amino acid sequences for six of the first seven amino-terminal residues, but are identical in primary structure from the 8th through the 24th amino acid residue. The isomitogen containing four L subunits (L-PHAP) is a potent leukoagglutinin and mitogen that lacks hemagglutinating properties. The isomitogen made up of four R subunits (4R H-PHAP) is a potent hemagglutinin. The hybrid isomitogens consisting of varying proportions of the two subunits (3L-1R, 2L-2R, 1L-3R) are capable of causing mixed erythrocyte-lymphocyte agglutination. These studies provide a structural basis for explaining the differences in biological activities of the various PHAP isomitogens.