A structural model for elongation factor 1 (EF-1) and phosphorylation by protein kinase CKII

Abstract

EF-1a binds aminoacyl-tRNA to the ribosome with the hydrolysis of GTP; the βγδ 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 β, γ, and δ 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 β and γ 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 β subunit and reduced the Km, while addition of α to β or the βγ complex inhibited phosphorylation by CKII. However, a had little effect on phosphorylation of δ. Thus, the β and δ subunits in EF-1 were differentially phosphorylated by CKII, in that phosphorylation of β was altered by association with other subunits, while the site on δ 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 (α2βγ2δ)2 is proposed and discussed.