Recombinations of subunits of Phaseolus vulgaris isolectins

Phaseolus vulgaris phytohemagglutinin is formed in vivo by the combination of erythrocyte (E)-reactive and lymphocyte (L)-reactive subunits into five tetrameric isolectins:L4,L3E1, L2E2, L1E3, and E4. Evidence for phytohemagglutinin subunit structure is obtained by in vitro dissociation of native isolectins in 6 M guanidine HCl followed by removal of dissociating agents to allow subunit recombination. Dissociation and recombination of L4 yielded a single protein, electrophoretically indistinguishable from the native L4. Similar treatment of E4 also yielded a single protein indistinguishable from native E4. Treatment of L3E1, L2E2, L1E3, or a mixture of L4 and E4, yielded five distinct proteins electrophoretically similar to all five native phytohemagglutinin isolectins. Milligram quantities of all five recombinant isolectins were prepared either from L2E2 or a mixture of L4 and L1E3 proportioned to yield equimolar quantitives of the two subunits on dissociation. The recombinant isolectins were purified by affinity and SP-Sephadex ion exchange chromatography. Electrophoretic and chromatographic properties and the erythroagglutinating and mitogenic activities of recombinant isolectins were essentially identical with the native isolectins. The inclusion of 125I-labeled L4 in the dissociation results in a distribution of 125I-labeled L subunit among the purified recombinant isolectins proportional to their proposed subunit structures.