Inhibition of rabbit reticulocyte guanine nucleotide exchange factor activity by heparin and its reversal by polyamines.

We have previously demonstrated that phosphorylation of the 82-kDa subunit of the guanine nucleotide exchange factor, eIF-2B, by casein kinases I and II stimulates eIF-2B activity. In the present report, we show that heparin binds to eIF-2B (KD approximately 0.3 microM) and inhibits the GDP/GTP exchange activity of eIF-2B. The effect is dose dependent, with half-maximal inhibition (IC50) achieved at 0.03 microM heparin. Although spermidine alone does not stimulate or inhibit guanine nucleotide exchange, it reverses the inhibitory effect of heparin up to 90 percent (IC50 = 0.22 mM). ATP and NADPH which interact with the 65 and 55-kDa subunits also partially relieve heparin inhibition of nucleotide exchange, and the effects of ATP and NADPH are additive. By using a gel overlay technique, we demonstrate that 125I-heparin binds to the 82, 65 and 55-kDa subunits of eIF-2B.