ERK3 is an atypical
mitogen-activated protein kinase (MAPK) that has recently gained interest for its role in promoting
cancer cell migration and invasion. However, the molecular regulation of ERK3 functions in
cancer cells is largely unknown. ERK3 has a single phospho-acceptor site (Ser189) in its activation motif rather than the TXY conserved in conventional MAPKs such as ERK1/2. Although dual phosphorylation of the TXY motif is known to be critical for the activation of conventional MAPKs, the role of Ser189 phosphorylation in ERK3 activity and its function in
cancer cells remain elusive. In this study, we revealed that activation loop phosphorylation is important for ERK3 in promoting
cancer cell invasiveness, as the S189A mutation greatly decreased the ability of ERK3 to promote migration and invasion of
lung cancer cells. Interestingly, a catalytically inactive ERK3 mutant was still capable of increasing migration and invasion, although to a lesser extent compared with WT ERK3, suggesting that ERK3 promotes
cancer cell invasiveness by both
kinase-dependent and
kinase-independent mechanisms. To elucidate how the S189A mutation reduces the invasiveness-promoting ability of ERK3, we tested its effect on the
kinase activity of ERK3 toward
steroid receptor coactivator 3 (SRC3), a recently identified substrate of ERK3 critical for
cancer cell invasiveness. Compared with ERK3, ERK3-S189A exhibited a dramatic decrease in
kinase activity toward SRC3 and a concomitantly reduced ability to stimulate
matrix metalloproteinase expression. Taken together, our study unravels the importance of Ser189 phosphorylation for intramolecular regulation of
ERK3 kinase activity and invasiveness-promoting ability in
lung cancer cells.