A factor protecting mammalian [ 75Se]SeCys-tRNA is different from EF-1α

Abstract

In Escherichia coli, an elongation factor (EF-Tu-like) specific to SeCys-tRNA, SELB, has been identified; however, a mammalian counterpart of SELB has not been reported to date. We searched for and found this factor in bovine liver extracts using the assay of [ 75Se]SeCys-tRNA protecting activity against alkaline hydrolysis (SePF activity). We found SePF activity in the protein extracts of the precipitate (microsomal fraction) collected at 150,000 × g from bovine liver. The proteins were separated by Sephacryl S-300 chromatography, and the SePF and EF-1α activities were found in the same fraction, indicating that SePF and EF-1α have the same molecular mass (approximately 50 kDa). We then chromatographed this active fraction using CM-Sephadex C-25 columns. The SePF activity was eluted after the peak of EF-1α activity. This result indicated that this SePF activity was not dependent on EF-1α. In addition to performing these two chromatographies, we investigated pure EF-1α from Bombyx mori but could not detect any SePF activity in B. mori EF-1α. Thus we showed that the SePF activity in bovine liver differs from that of EF-1α in eukaryotes. Therefore the factor protecting [ 75Se]SeCys-tRNA in bovine liver is not EF-1α and must be a SELB-like factor.