Catalytic subunit of the guanine nucleotide exchange factor (GEF) (eEF1B subcomplex) of the eukaryotic elongation factor 1 complex (eEF1). Stimulates the exchange of GDP for GTP on elongation factor 1A (eEF1A), probably by displacing GDP from the nucleotide binding pocket in eEF1A. The 30-fold higher concentration of GTP compared to GDP in cells favors the formation of eEF1A-GTP, which rapidly forms a ternary complex with aminoacyl-tRNA that in turn displaces eEF1B from the complex. Protein biosynthesis; polypeptide chain elongation. The eukaryotic elongation factor 1 complex (eEF1) is probably a heterohexamer. Two trimeric complexes, each composed of eEF1A (TEF1 or TEF2), eEF1Balpha (EFB1) and eEF1Bgamma (CAM1 or TEF4), are probably dimerized via the eF1Bgamma subunits. eEF1Balpha interacts directly with eEF1A. eEF1Balpha and eEF1Bgamma form the eEF1B subcomplex with the GEF activity. The C-terminus (pos. 119-205) exhibits guanine nucleotide exchange activity. S-thiolated in response to oxidative stress, probably inhibiting the protein and causing a reduction in protein synthesis. Belongs to the EF-1-beta/EF-1-delta family.