Quaternary Protein Model of the Barley MKK3 Enzyme in Complex with Its Candidate Upstream MAP3K Kinase and Functional Implications

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.