Abstract
Since the discovery of selenocysteine as the 21st amino acid
considerable progress has been made in elucidating the system
responsible for its insertion into proteins. Elongation factor SELB,
whose amino-terminal part shows homology to EF-Tu, was found to be the
key component mediating delivery of selenocysteyl-tRNA(Sec) to the
ribosomal A site. It exhibits a distinct tertiary structure comprising
binding sites for guanosine nucleotides, the cognate tRNA, an mRNA
secondary structure (SECIS element) and presumably ribosomal components.
The kinetics of interaction of SELB with its ligands have been studied
in detail. GDP was found to bind with about 20-fold lower affinity than
GTP and to be in rapid exchange, which obviates the need for a guanosine
nucleotide exchange factor. The affinity of SELB for the SECIS element
is in the range of 1 nM and further increases upon binding of
selenocysteyl-tRNA(Sec) to the protein. This supports the model that
SELB forms a tight quaternary complex on the SECIS element which is
loosened after insertion of the tRNA into the ribosomal A site and the
concomitant hydrolysis of GTP.
