The RNA binding domain of SBP2 is the Sec‐specific elongation factor binding site and the central regulator of the Sec incorporation mechanism

The co‐translational insertion of the 21st amino acid, selenocysteine (Sec), into 25 human proteins provides the bulk of a cell's catalytic activity against oxidative stress, which is causative in several pathological conditions such as cancer. During protein synthesis, selenocysteine gets delivered and incorporated at in‐frame UGA codons by the Sec‐specific elongation factor (eEFSec). A Sec Insertion Sequence (SECIS) element found in the 3′ UTR of all selenoprotein mRNAs and the SECIS binding protein 2 (SBP2) are also required for Sec incorporation. Previous studies have revealed the importance of SBP2 for SECIS binding and Sec incorporation. However, the mechanism by which the SBP2/SECIS complex activates eEFSec during Sec incorporation is still unknown. In our study we discovered that the RNA binding domain (RBD) of SBP2 is sufficient to form a complex with eEFSec in a SECIS‐dependent manner. Alanine scanning mutagenesis was performed at conserved regions in the RBD in order to identify the putative eEFSec binding site, and binding data was correlated with in vitro Sec incorporation activity. Preliminary results support a model in which the SECIS element induces a conformational change within the RBD of SBP2 that allows it to interact and activate eEFSec for the insertion of selenocysteine during protein synthesis. Funding source: NIH Biotechnology Training Program, NSF IGERT Biointerface and the NIGMS