PMC

Overall structure of A. aeolicus SelA. PLPs are represented as sphere models. The 10 subunits are shown in different colors. The disordered regions in the N-terminal domains are colored gray.

(A) The catalytic site of SelA-ΔN in complex with thiosulfate. PLP and thiosulfate ions (TS1 to TS3) are shown. TS1 mimics the substrate selenophosphate (Se-P). The subunits of the intimate dimer A•B are colored as in , whereas subunit J is colored gray. (B) Docking model of the Ser-ligated terminal tRNA^Sec nucleotide (A76) and Se-P. (C) SelA surface, showing that A76 binds to the boundary of subunits J and A. The docked A76-Ser and G73 of tRNA^Sec are connected through C74-C75.

In the waiting state, the PLP forms an internal aldimine with Lys²⁸⁵ (A). The amino group of Ser-tRNA^Sec replaces Lys²⁸⁵ to form an external aldimine (B), followed by deprotonation of the α position of the Ser moiety (C). Lys²⁸⁵ protonates and eliminates the β hydroxyl group to form a 2-aminoacrylyl moiety (D). The nucleophilic addition of selenophosphate generates a Se-phosphorylated Sec moiety (E), which is hydrolyzed to a Sec moiety and a phosphate (F). Lastly, the α position of the Sec moiety is protonated (G), and Sec-tRNA^Sec is released (H).

(A) Overall structure of A. aeolicus SelA complexed with T. tengcongensis tRNA^Sec (colored brown). (B) Interaction of tRNA^Sec with intimate dimers I•J and A•B (close-up view). The omit F_obs-F_calc map (3.0 sigma level) of the tRNA^Sec is shown in a blue mesh. The putative position of the CCA region is shown as a dashed line. (C) A schematic diagram of the interaction of tRNA^Sec with dimers I•J and A•B. (D and E) Discrimination of tRNA^Sec from tRNA^Ser by SelA. T. thermophilus tRNA^Ser [PDB ID 1SER ()] was docked to the SelA•tRNA^Sec complex by superimposing the backbone phosphorus atoms of the T arm. The tRNA^Sec surface is complementary to that of the SelA N-terminal domain (D), whereas that of tRNA^Ser is not complementary (E).