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.

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