Light-Oxygen-Voltage (LOV)-sensing Domains: Activation Mechanism and Optogenetic Stimulation

The light-oxygen-voltage (LOV) domains of phototropins emerged as essential constituents of light-sensitive proteins, helping initiate blue light-triggered responses. Moreover, these domains have been identified across all kingdoms of life. LOV domains utilize flavin nucleotides as co-factors and undergo structural rearrangements upon exposure to blue light, which activates an effector domain that executes the final output of the photoreaction. LOV domains are versatile photoreceptors that play critical roles in cellular signaling and environmental adaptation; additionally, they can noninvasively sense and control intracellular processes with high spatiotemporal precision, making them ideal candidates for use in optogenetics, where a light signal is linked to a cellular process through a photoreceptor. The ongoing development of LOV-based optogenetic tools, driven by advances in structural biology, spectroscopy, computational methods, and synthetic biology, has the potential to revolutionize the study of biological systems and enable the development of novel therapeutic strategies.

Crystallization of a human galectin-3 variant with two ordered segments in the shortened N-terminal tail

Among members of the family of adhesion/growth-regulatory galectins, galectin-3 (Gal-3) bears a unique modular architecture. A N-terminal tail (NT) consisting of the N-terminal segment (NTS) and nine collagen-like repeats is linked to the canonical lectin domain. In contrast to bivalent proto- and tandem-repeat-type galectins, Gal-3 is monomeric in solution, capable to self-associate in the presence of bi- to multivalent ligands, and the NTS is involved in cellular compartmentalization. Since no crystallographic information on Gal-3 beyond the lectin domain is available, we used a shortened variant with NTS and repeats VII-IX. This protein crystallized as tetramers with contacts between the lectin domains. The region from Tyr101 (in repeat IX) to Leu114 (in the CRD) formed a hairpin. The NTS extends the canonical β-sheet of F1-F5 strands with two new β-strands on the F face. Together, crystallographic and SAXS data reveal a mode of intramolecular structure building involving the highly flexible Gal-3’s NT.

Preliminary X-ray crystallographic analysis of an engineered variant of human chimera-type galectin-3 with a shortened N-terminal domain

How lectins translate sugar-encoded information into cellular effects not only depends on glycan recognition. Other domains of the protein can contribute to the functional profile of a lectin. Human galectin-3 (Gal-3), an adhesion/growth-regulatory galectin, is composed of three different domains and is thus called a chimera-type protein. In addition to the carbohydrate-recognition domain, this lectin encompasses an N-terminal domain consisting of a peptide harbouring two phosphorylation sites and nine non-triple-helical collagen-like repeats. This region plays an as yet structurally undefined role in Gal-3 aggregation and ligand recognition. To date, crystallization of full-length Gal-3 has not been achieved. With the aim of providing structural insights into this modular organization, a Gal-3 variant was crystallized maintaining the terminal peptide and three of the nine collagen-like repeats. The crystals belonged to the orthorhombic space group P212121, with unit-cell parameters a = 94.04, b = 97.96, c = 236.20 Å, and diffracted to a resolution of 3.3 Å.