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Human biliverdin reductase is an ERK activator; hBVR is an ERK nuclear transporter and is required for MAPK signaling

Lerner-Marmarosh, Nicole, Miralem, Tihomir, Gibbs, Peter E.M., Maines, Mahin D.
Proceedings of the National Academy of Sciences of the United States of America 2008 v.105 no.19 pp. 6870-6875
gene activation, humans, in vitro studies, mitogen-activated protein kinase, mutants, nuclear localization signals, peptides, phosphorylation, protein transport, protein-protein interactions, transcription (genetics), transfection
Activation of the MEK/ERK/Elk-signaling cascade is a mechanism for relaying mitogenic and stress stimuli for gene activation. MEK1 is the proximate kinase for activation of ERK1/2, and nuclear targeting of ERK1/2 is obligatory for Elk1 transcriptional activity. Human biliverdin reductase (hBVR) is a recently described Ser/Thr/Tyr kinase in the MAPK insulin/insulin-like growth factor 1 (IGF1)-signaling cascade. Using 293A cells and in vitro experiments, we detail the formation of a ternary complex of MEK/ERK/hBVR, activation of MEK1 and ERK1/2 kinase activities by hBVR, and phosphorylation of hBVR by ERK1/2. hBVR is nearly as effective as IGF1 in activating ERK; intact hBVR ATP-binding domain is necessary for Elk1 activation, whereas protein-protein interaction is the basis for hBVR activation of MEK1 and ERK. The two MAPK docking consensus sequences present in hBVR, F¹⁶²GFP and K²⁷⁵KRILHCLGL (C- and D-box, respectively), are ERK interactive sites; interaction at each site is critical for ERK/Elk1 activation. Transfection with mutant hBVR-P¹⁶⁵ or peptides corresponding to the C- or D-box blocked activation of ERK by IGF1. Transfection with D-box mutant hBVR prevented the activation of ERK by wild-type protein and dramatically decreased Elk1 transcriptional activity. hBVR is a nuclear transporter of ERK; experiments with hBVR nuclear export signal (NES) and nuclear localization signal (NLS) mutants demonstrated its critical role in the nuclear localization of IGF-stimulated ERK for Elk1 activation. These findings, together with observations that si-hBVR blocked activation of ERK and Elk1 by IGF1 and prevented formation of ternary complex between MEK/ERK/hBVR, define the critical role of hBVR in ERK signaling and nuclear functions of the kinase.