A role for the carbohydrate-binding module (CBM) in regulatory SnRK1 subunits: the effect of maltose on SnRK1 activity.

SnRK1 is a protein kinase complex that is involved in several aspects of plant growth and development. There are published data indicative of a participation of SnRK1 in the regulation of the synthesis and degradation of starch, although the molecular mechanism is not known. In this work, we performed electron microscopy to explore the in vivo localization of the regulatory and catalytic subunits that constitute the SnRK1 complex. The results indicated that all the subunits are present in the chloroplast and, in particular, the SnRK1 βγ and SnRK1 β3 subunits are associated with starch. Furthermore, the regulatory subunits bind maltose, a relevant product of starch degradation. The kinase activity of immunoprecipitated complexes containing the βγ regulatory subunit was positively regulated by maltose only in the complexes obtained from Arabidopsis leaves collected at dusk. Recombinant complexes with the SnRK1α1 catalytic subunit, SnRK1βγ and three different β subunits showed that maltose only had an effect on a complex formed with the β3 subunit. Truncation of the CBM domain form SnRK1 βγ abolished the maltose activation of the complex and the activity was significantly reduced, indicating that the CBM is a positive regulator of SnRK1. A model of the SnRK1α1/βγ/β3 complex suggests the presence of two putative maltose-binding sites, both involving ligand interactions with the βγ subunit and the α subunit.