Recent studies have identified a novel interkingdom signaling circuit, via plant signaling molecules, and a bacterial sub-family of LuxR proteins, bridging eukaryotes and prokaryotes. Indeed pivotal plant-bacteria interactions are regulated by the so called Plant Associated Bacteria (PAB) LuxR solo regulators that, although closely related to the quorum sensing (QS) LuxR family, do not bind or respond to canonical quorum sensing N-acyl homoserine lactones (AHLs), but only to specific host plant signal molecules. The large body of structural data available for several members of the QS LuxR family complexed with different classes of ligands (AHLs and other compounds), has been exploited to dissect the cartography of their regulatory domains through structure-based multiple sequence alignments, structural superimposition and a comparative analysis of the contact residues involved in ligand binding. In the absence of experimentally determined structures of members of the PAB LuxR solos subfamily, an homology model of its prototype OryR is presented, aiming to elucidate the architecture of its ligand-binding site. The obtained model, in combination with the cartography of the regulatory domains of the homologous QS LuxRs, provides novel insights into the 3D structure of its ligand-binding site and unveils the probable molecular determinants responsible for differences in selectivity towards specific host plant signal molecules, rather than to canonical QS compounds.

Dystroglycan (DG), composed of α and β subunits, belongs to the dystrophin‐associated glycoprotein complex. α‐DGis an extracellular matrix protein that undergoes a complex post‐translational glycosylation process. The bifunctional glycosyltransferase like‐acetylglucosaminyltransferase (LARGE) plays a crucial role in the maturation of α‐DG, enabling its binding to laminin. We have already structurally analyzed the N‐terminal region of murine α‐DG (α‐DG‐Nt) and of a pathological single point mutant that may affect recognition of LARGE, although the structural features of the potential interaction betweenLARGEandDGremain elusive. We now report on the crystal structure of the wild‐type human α‐DG‐Nt that has allowed us to assess the reliability of our murine crystallographic structure as a α‐DG‐Nt general model. Moreover, we address for the first time both structures in solution. Interestingly, small‐angle X‐ray scattering (SAXS) reveals the existence of two main protein conformations ensembles. The predominant species is reminiscent of the crystal structure, while the less populated one assumes a more extended fold. A comparative analysis of the human and murine α‐DG‐Nt solution structures reveals that the two proteins share a common interdomain flexibility and population distribution of the two conformers. This is confirmed by the very similar stability displayed by the two orthologs as assessed by biochemical and biophysical experiments. These results highlight the need to take into account the molecular plasticity of α‐DG‐Nt in solution, as it can play an important role in the functional interactions with other binding partners.

Dystroglycan (DG) is a highly glycosylated protein complex that links the cytoskeleton with the extracellular matrix, mediating fundamental physiological functions such as mechanical stability of tissues, matrix organization and cell polarity. A crucial role in the glycosylation of the DG α subunit is played by its own N-terminal region that is required by the glycosyltransferase LARGE. Alteration in this O-glycosylation deeply impairs the high affinity binding to other extracellular matrix proteins such as laminins. Recently, three missense mutations in the gene encoding DG, mapped in the α-DG N-terminal region, were found to be responsible for hypoglycosylated states, causing congenital diseases of different severity referred as primary dystroglycanopaties.To gain insight on the molecular basis of these disorders, we investigated the crystallographic and solution structures of these pathological point mutants, namely V72I, D109N and T190M. Small Angle X-ray Scattering analysis reveals that these mutations affect the structures in solution, altering the distribution between compact and more elongated conformations. These results, supported by biochemical and biophysical assays, point to an altered structural flexibility of the mutant α-DG N-terminal region that may have repercussions on its interaction with LARGE and/or other DG-modifying enzymes, eventually reducing their catalytic efficiency.

On the cover: This cover image, by Giulia Signorino et al., is based on the Research Article A dystroglycan mutation (p.Cys667Phe) associated to muscle-eye-brain disease with multicystic leucodystrophy results in ER-retention of the mutant protein, Pages 266–280. DOI: 10.1002/humu.23370.