Investigation of Escherichia coli Selenocysteine Synthase (SelA) Complex Formation Using Cryo-Electron Microscopy (Cryo-EM)

Abstract

Incorporation of selenocysteine (Sec U) into proteins is directed by a in-frame UGA codon in all domains of life. In Bacteria, Sec biosynthesis and incorporation involves the interaction of Selenocysteine Synthase (SelA), tRNA (SelC or tRNA Sec ), Selenophosphate Synthetase (SPS), a specific elongation factor known as SelB and the specific mRNA structure SElenocysteine Insertion Sequence (SECIS), forming a complex molecular machinery. SelA is a homodecamer complex responsible for Ser-Sec conversion from selenophosphate delivered by SPS and seryl-tRNA sec , which differs from seryl-tRNA ser by its long variable arm and the UGA-codon. The specific mRNA sequence known as SElenoCysteine Insertion Sequence forms a hairpin-like secondary structure and is recognized by SelB for Sec incorporation in the nascent peptide [1-3]. Since selenium compounds are highly toxic in cellular environment, selenium association with proteins complexes throughout its metabolism is suggested to be essential for cell survival. However, macromolecular interactions between the different proteins have not yet been characterized.