Background: The Ras/Raf/MEK/ERK signaling pathway is involved in essential cell processes and it is abnormally activated in ~30 % of cancers and cognitive disorders. Two ERK isoforms have been described, ERK1 and ERK2; ERK2 being regarded by many as essential due to the embryonic lethality of ERK2 knock-out mice, whereas mice lacking ERK1 are viable and fertile. The controversial question of why we have two ERKs and whether they have differential functions or display functional redundancy has not yet been resolved.
Results: To investigate this question we used a novel approach based on comparing the evolution of ERK isoforms' sequences and protein expression across vertebrates. We gathered and cloned erk1 and erk2 coding sequences and we examined protein expression of isoforms in brain extracts in all major clades of vertebrate evolution. For the first time, we measured each isoforms' relative protein level in phylogenetically distant animals using anti-phospho antibodies targeting active ERKs. We demonstrate that squamates (lizards, snakes and geckos), despite having both genes, do not express ERK2 protein whereas other tetrapods either do not express ERK1 protein or have lost the erk1 gene. To demonstrate the unexpected squamates' lack of ERK2 expression, we targeted each ERK isoform in lizard primary fibroblasts by specific siRNA-mediated knockdown. We also found that undetectable expression of ERK2 in lizard is compensated by a greater strength of lizard's erk1 promoter. Finally, phylogenetic analysis revealed that ERK1 amino acids sequences evolve faster than ERK2's likely due to genomic factors, including a large difference in gene size, rather than from functional differences since amino acids essential for function are kept invariant.
Conclusions: ERK isoforms appeared by a single gene duplication at the onset of vertebrate evolution at least 400 Mya. Our results demonstrate that tetrapods can live by expressing either one or both ERK isoforms, supporting the notion that ERK1/2 act interchangeably. Substrate recognition sites and catalytic cleft are nearly invariant in all vertebrate ERKs further suggesting functional redundancy. We suggest that future ERK research should shift towards understanding the role and regulation of total ERK quantity, especially in light of newly described erk2 gene amplification identified in tumors.