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

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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.