Maize CDKA2;1a and CDKB1;1 kinases have different requirements for their activation and participate in substrate recognition.

Cyclin-dependent kinases (CDKs), in association with cyclins, control cell cycle progression by phosphorylating a large number of substrates. In animals, activation of CDKs regularly requires both the association with a cyclin and then phosphorylation of a highly conserved threonine residue in the CDK activation loop (the classical mechanism), mediated by a CDK-activating kinase (CAK). In addition to this typical mechanism of activation, some CDKs can also be activated by the association of a cyclin to a monomeric CDK previously phosphorylated by CAK although not all CDKs can be activated by this mechanism. In animals and yeast, cyclin, in addition to being required for CDK activation, provides substrate specificity to the cyclin/CDK complex; however, in plants both the mechanisms of CDKs activation and the relevance of the CDK-associated cyclin for substrate targeting have been poorly studied. In this work, by co-expressing proteins in E. coli, we studied maize CDKA2;1a and CDKB1;1, two of the main types of CDKs that control the cell cycle in plants. These kinases could be activated by the classical mechanism and by the association of CycD2;2a to a phosphorylated intermediate in its activation loop, a previously unproven mechanism for the activation of plant CDKs. Unlike CDKA2;1a, CDKB1;1 did not require CAK for its activation, since it autophosphorylated in its activation loop. Phosphorylation of CDKB1;1 and association of CycD2;2 was not enough for its full activation as association of maize CKS, a scaffolding protein, differentially stimulated substrate phosphorylation. Our results suggest that both CDKs participate in substrate recognition.

Unlocking the structural features for the xylobiohydrolase activity of an unusual GH11 member identified in a compost-derived consortium.

The heteropolysaccharide xylan is a valuable source of sustainable chemicals and materials from renewable biomass sources. A complete hydrolysis of this major hemicellulose component requires a diverse set of enzymes including endo-ß-1,4-xylanases, ß-xylosidases, acetylxylan esterases, α-l-arabinofuranosidases, and α-glucuronidases. Notably, the most studied xylanases from glycoside hydrolase family 11 (GH11) have exclusively been endo-ß-1,4- and ß-1,3-xylanases. However, a recent analysis of a metatranscriptome library from a microbial lignocellulose community revealed GH11 enzymes capable of releasing solely xylobiose from xylan. Although initial biochemical studies clearly indicated their xylobiohydrolase mode of action, the structural features that drive this new activity still remained unclear. It was also not clear whether the enzymes acted on the reducing or nonreducing end of the substrate. Here, we solved the crystal structure of MetXyn11 in the apo and xylobiose-bound forms. The structure of MetXyn11 revealed the molecular features that explain the observed pattern on xylooligosaccharides released by this nonreducing end xylobiohydrolase.