IDENTIFICATION OF A SUBSTRATE SITE FOR TRANSGLUTAMINASES ON THE HUMANPROTEIN-SYNTHESIS INITIATION-FACTOR 5A

Protein synthesis initiation factor 5A (eIF-5A) from human erythrocyte s was found to be a substrate for both plasma transglutaminase (Factor XIIIa) and guinea pig liver transglutaminase (GPLTG). When purified e IF-5A was incubated with GPLTG or Factor XIIIa in the presence of succ inylated beta-casein, a covalent complex was identified. By isolating and analysing the product of the transglutaminases (TGases) reaction, the site of modification on eIF-SA has been identified as the unique a mino acid hypusine. The complex beta-casein eIF-5A was enzymically dig ested with proteinases and the predicted covalent cross-link of gamma- glutamyl-<(omega)over bar>-hypusine was isolated from the digests by i on-exchange chromatography and purified by reversed-phase h.p.l.c. Aci d hydrolysis of the purified dipeptide yielded equimolar amounts of hy pusine and glutamic acid, Furthermore, fast atom bombardment m.s. anal ysis confirmed the isomer assignment to be gamma-glutamyl-(omega)over bar>-hypusine. These data indicate that hypusine-50 of the eIF-5A chai n functions as acyl acceptor substrate for TGases, and reveal that eIF -5A may be cross-linked to intracellular proteins by TGases, Because t he precise function of eIF-5A is still unknown, our results appear par ticularly stimulating in the light of the recent finding of a new biol ogical role for this protein as a cellular factor binding specifically to the human immunodeficiency virus-1 Rev activation domain [Ruhl, Hi mmelspach, Bahr, Hammerschmid, Jaksche, WoIff; Auschauer, Farrington, Probst, Bevec and Hauber (1993) J. Cell Biol. 123, 1309-1320].