EF-1alpha binds aminoacyl-tRNA to the ribosome with the hydrolysis of GTP; the betagammadelta complex facilitates the exchange of GDP for GTP to initiate another round of elongation. To examine the subunit structure of EF-1 and phosphorylation by protein kinase CKII, recombinant beta, gamma, and delta subunits from rabbit were expressed in E. coli and the subunits were reconstituted into partial and complete complexes and analyzed by gel filtration. To determine the availability of the beta and delta subunits for phosphorylation by CKII, the subunits and the reconstituted complexes were examined as substrates for CKII. Formation of the nucleotide exchange complex increased the rate of phosphorylation of the beta subunit and reduced the Km, while addition of alpha to beta or the betagammacomplex inhibited phosphorylation by CKII. However, alpha had little effect on phosphorylation of delta. Thus, the beta and delta subunits in EF-1 were differentially phosphorylated by CKII, in that phosphorylation of beta was altered by association with other subunits, while the site on delta was always available for phosphorylation by CKII. From the availability of the subunits for phosphorylation by CKII and the composition of the reconstituted partial and complete complexes, a model for the subunit structure of EF-1 consisting of(alpha2betagamma2delta)2 is proposed and discussed.