Fungal galectin CGL2
Coprinopsis cinerea (Inky cap fungus)
Lectins are proteins that bind carbohydrates. Galectins are a family of animal lectins that have galactose-binding ability and amino acid sequences that are characteristic to galectins. Galectins are widespread in the animal kingdom and share primary structural homology in their ~130 amino acid carbohydrate recognition domains (CRDs). They do not require Ca2+ or any other metal for their activity. Galectins have many features that are common to cytoplasmic proteins, like no disulfide bridges, no sugar chains, no signal sequences and acetylation of N-terminal amino acids. However, they have been found to be localized in the nuclei, on cell surfaces and in extracellular spaces along with the cytoplasm. They have been implicated to be involved in a number of different, unrelated processes like pre-mRNA splicing, cell-signaling, extracellular cell adhesion, cell matrix interaction, embryogenesis and tumorigenesis among others. However, no explicit function has yet been identified for galectins neither is it understood how they discriminate between different substrates in terms of carbohydrate recognition.
The structure shown here is that of fungal galectin CGL2. CGL2 is a tetramer with each monomer consisting of two anti-parallel beta-sheets forming a beta-sandwich, commonly known as a jelly-roll. The C-terminal extensions of the top two subunits hook into those of the bottom two subunits. The substrate binding site is present on the concave side of the beta-sandwich. Thus the tetramer has four binding sites in orthogonal orientation which seem to impact substrate binding and selectivity. The authors propose that CGL2 achieves substrate specificity by allowing extended binding into a cleft beyond the actual binding site.
Protein Data Bank (PDB)
author: Ashwini Patil