An AUG Codon Conserved for Protein Function Rather than Translational Initiation: The Story of the Protein sElk1

Elk1 belongs to the ternary complex (TCF) subfamily of the ETS-domain transcription factors. Several studies have implicated an important function for Elk1 in the CNS including synaptic plasticity and cell differentiation. Whilst studying ELK1 gene expression in rat brain a 54 aa N-terminally truncated isoform lacking the DBD was observed on immunoblots. A similar protein was also detected in NGF differentiated PC12 cells. It was proposed that this protein, referred to as sElk1, arose due to a de-novo initiation event at the second AUG codon on the Elk1 ORF. Transient over-expression of sElk1 potentiated neurite growth in the PC12 model and induced differentiation in the absence of NGF, leading to the proposition that it may have a specific function in the CNS. Here we report on the translational expression from the mouse and rat transcript and compare it with our earlier published work on human. Results demonstrate that the previously observed sElk1 protein is a non-specific band arising from the antibody employed. The tight conservation of the internal AUG reported to drive sElk1 expression is in fact coupled to Elk1 protein function, a result consistent with the Elk1-SRE crystal structure. It is also supported by the observed conservation of this methionine in the DBD of all ETS transcription factors independent of the N- or C-terminal positioning of this domain. Reporter assays demonstrate that elements both within the 5′UTR and downstream of the AUG(Elk1) serve to limit 40S access to the AUG(sElk1) codon.