Virtuelle (Nano) Welten PDB Molecule of the Month 104: Selenocysteine Synthase

Specifying Selenium

Elongation factor SelB is the protein that decides when to add a selenocysteine at UGA codons instead of stopping. It recognizes a special "selenocysteine insertion sequence" that is found after the UGA codon and forms a distinctive hairpin loop in the messenger RNA. PDB entry 1wb1, shown on the left, shows the whole protein. The large portion at the top binds to GTP (shown in orange) and delivers the selenocysteine tRNA to the ribosome. It is similar to elongation factor Tu, which delivers the twenty normal tRNA molecules to the ribosome. This is attached to a flexible tail that binds to the insertion sequence in the messenger RNA. The structure on the right (PDB entry 2uwm) includes this flexible tail of SelB along with a piece of the messenger RNA (in red and orange). The protein recognizes a guanine base that is flipped out in the loop, along with the overall shape of the hairpin. To see the atomic details of this recognition, take a look at the page at Proteopedia.

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