Figure 5.

Amino acid insertion at UGA codons SEC3, SEC4 and SEC5 in SELENOP. HepG2 cells were treated with 100 nM Se, 200 µg/mL G418 or their combination. SELENOP was purified by immuno-affinity and subjected to LC-MS/MS analysis. (A) Sec in SELENOP was detected almost exclusively at the positions SEC3, SEC4 and SEC5 when cells were supplemented with selenite. (B) The pattern of amino acids inserted at the three Sec codons varied strongly when cells were grown in the presence of 100 nM selenite and 200 µg/mL G418. (C) SELENOP synthesized in the absence of supplemental selenite but in presence of 200 µg/mL G418 was devoid of Sec residues at the three positions available for analysis (SEC3-5). The amino acids replacing Sec were mainly tryptophan (W), cysteine (C) and arginine (R), but their relative proportions were Sec-codon specific.