Usually, ribosomes halt on mRNA strands when they encounter a UGA codon. But for some mRNAs in all forms of life, the genetic code is recoded to incorporate selenocysteine instead. This process essential to life, that interprets in-frame UGA stop codons as selenocysteine is poorly understood.
Their findings were published last week in and article in the journal
“This work revealed structures that had never before been seen, some of which are unique in all of biology,” said Paul Copeland, a professor in the Department of Biochemistry and Molecular Biology at Rutgers Robert Wood Johnson Medical School, who is an author of the study.
Using specialized cryo-electron microscopy, stop-motion animation, and computational tools, the team discerned how the translational machinery works to dictate the function of the ribosome in incorporating selenocysteine.
“This amino acid gets attached to a unique RNA molecule that has to be carried to the ribosome via a unique protein factor,” said Copeland, whose lab has spent the past two decades unraveling the selenocysteine incorporation process. “And all of this evolved in humans specifically to allow selenium to be incorporated into this handful of proteins.”
The authors demonstrate that during the process of selenocysteine incorporation, an RNA-protein complex forms between the noncoding selenocysteine-insertion sequence (SECIS) in the selenoprotein mRNA, SECIS-binding protein 2 (SBP2), and 40S ribosomal subunit, which enables the selenocysteine -specific translation of elongation factor, eEFSec, to deliver the unique amino acid. The researchers show that the translation elongation factor and SBP2 does not physically interact but use these carboxyl tails to engage opposite ends of the noncoding selenocysteine-insertion sequence. At the same time, the ribosomal protein eS31 binds selenocysteine-specific transfer RNA (tRNASec) and SBP2, which increases the stability of the complex.
The authors also show that the elongation factor for selenocysteine eEFSec, can also engage another amino acid, L-serine, which can mis-incorporate serine at selenocysteine-UGA codons.
Copeland hopes to one day be able to specifically regulate the expression of selenoproteins in vivo. To achieve this, his team continues to probe into the factors that contribute to selenocysteine incorporation in the zebrafish model.