Sequence- and position-dependent tagging protects extracellular-regulated kinase 3 protein from 26S proteasome-mediated degradation.

Extracellular-regulated kinase 3, an atypical member of the mitogen-activated protein kinase subfamily of extracellular-regulated kinases, was originally identified in 1991. Little is known about the biochemical properties, regulation, and biological functions of this protein kinase, partially due to the unstable nature of endogenous and low ectopical expression level of the protein. Here, we report that a single C-terminal c-myc tag increases the half-life of ectopic expressed tagged extracellular-regulated kinase 3 approximately four times compared to the reported 30 min half-life time for the endogenous protein and ectopic expressed extracellular-regulated kinase 3 deprived of its c-myc tag. These findings indicate that this C-terminal tag stabilizes the extracellular-regulated kinase 3. The stabilizing effect of the C-terminal c-myc tag is observed in all cell types tested, but is position- and tag sequence-dependent as neither N-terminal c-myc tag nor C-terminal HA tag stabilize the protein. The c-myc tag on extracellular-regulated kinase 3 did not interfere with its kinase activity, nor did it abrogate its ability to interacts with its bona fide substrate mitogen-activated protein kinase-activated protein kinase 5, indicating that tagging did not alter the known biological properties of the protein. Stabilization of the tagged extracellular-regulated kinase 3 protein probably results from reduced ubiquitination. In conclusion, position and sequence specific tagging should provide an easy and useful tool to generate a more stable protein that can functionally substitute the endogenous unstable protein. A stabilized variant may facilitate studies on the biological role of the protein.