- Article
The mitogen-activated protein kinase (MAPK) cascade is crucial in transmitting extracellular signals to the nucleus, where transcriptional patterns are adjusted in response to changing environmental conditions. In barley, a specific cascade enzyme, MKK3, mediates the duration of dormancy and hence the subsequent germination of the grain. Higher activity of this HvMKK3 isoenzyme is associated with decreased grain dormancy, but also with increased risks of yield losses through pre-harvest sprouting of the grain. Previous work has shown how the MKK3 isoenzyme docks with and phosphorylates its downstream substrate HvMAPK. Here, we used computational, analytical and phylogenetic methods to generate a quaternary structural model of the barley MKK3 enzyme with its candidate upstream MAP3K, working as a structurally consistent complex. Although this model awaits confirmation, it provides a structural rationale for the binding and subsequent hydrolysis of ATP by MAP3K and the phosphorylation of Thr residues in the downstream MKK3’s activation loop. This MAP3K enzyme is encoded by a single highly conserved gene on chromosome 5H in 76 wild and domesticated barley species. Potential functions of conserved protein sequence motifs in intrinsically disordered regions of the enzyme are considered in relation to the activation and turnover of the enzyme.
31 October 2026
![Schematic outline of the mitogen-activated kinase cascade. Following the binding of extracellular mitogens to their plasma membrane-bound receptor, a cascade at each level is initiated in which a mitogen-activated kinase kinase kinase (MAP3K) phosphorylates and activates a mitogen-activated kinase kinase (MAPKK), which in turn phosphorylates and activates a mitogen-activated kinase (MAPK). The activated MAPK subsequently switches on transcription factors in the nucleus. Typically, MAPK cascade proteins contain catalytic modules, additional folded domains, and extended intrinsically disordered regions (IDRs) present in MAPK and MAP3K enzymes. IDRs contain Gly, Ala, Asp, Glu, Pro, and Ser residue-rich regions (shown as random black structureless lines). Redrawn from Wang and Gou [6] and Hrmova et al. [5].](https://mdpi-res.com/kinasesphosphatases/kinasesphosphatases-04-00030/article_deploy/html/images/kinasesphosphatases-04-00030-g001-550.jpg)
![Representative chemical structures of oleanolic acid derivatives included in the quantitative SAR and target-oriented comparisons discussed in this review. (A) PTP1B/TCPTP-directed series, including OA-Br-1 (2b) and compound 8 from Song et al. [12], the bidesmosidic glycoconjugates 1h, 1i, and 1j from Liu et al. [11], and compounds 10p, 11p, 12m, and 13 from the PTP1B-directed series of Zhang et al. [10]. (B) RTK-oriented compounds, including the VEGFR2-active phenylurea derivatives I6 and I7 from Song et al. [21], the PDGFR-oriented analogues OA-4 (I6) and OA-6 (II4) from Meng et al. [43], and K73-03 from Zhou et al. [20]. The composite designations used for the PDGFR series distinguish OA-4 (I6) from the structurally unrelated I6 of the VEGFR2 series. (C) Dimeric OA derivatives representing phenotypic SAR and a FAK-oriented hypothesis. The common C-28-linked dimer scaffold is shown with variable linker L and C-3 substituent R. Compounds 2a, 2d, 2e, and 2f illustrate linker-length and linker-geometry effects, whereas 3a represents the C-3-acetylated analogue of 2a [13,14,15]. For compounds 2a, 2d, 2e, and 2f, R = H; for 3a, R = Ac. FAK involvement in the dimeric series is based on molecular docking and should be regarded as a target hypothesis requiring biochemical validation [14].](https://mdpi-res.com/kinasesphosphatases/kinasesphosphatases-04-00028/article_deploy/html/images/kinasesphosphatases-04-00028-ag-550.jpg)
