This review summarizes selected studies on nuclear complexes of galectin-3 (Gal3), a carbohydrate-binding protein found in the cytoplasm and nucleus of many cell types. The amino acid sequence delineates the polypeptide (Mr ~ 30,000) into two distinct domains: an intrinsically disordered NH2-terminal domain (NTD) containing tandem repeats of a proline- and glycine-rich motif (PGAYPG) and a COOH-terminal carbohydrate recognition domain (CRD) responsible for binding galactose-containing glycoconjugates. Depletion-reconstitution experiments in cell-free splicing assays document that Gal3 is a required pre-mRNA splicing factor. The protein is incorporated into spliceosomes via its association with the U1 small nuclear ribonucleoprotein (snRNP) complex and the U1-specific protein U1 70K is co-precipitated with Gal3 in a nuclease-sensitive manner. This association localizes Gal3 with snRNPs and other splicing factors (e.g. SC35) in nuclear speckles which are membrane-less subnuclear bodies formed by liquid-liquid phase separation. Recombinant NTD constructs and synthetic peptides containing the PGAYPG motif inhibit the splicing reaction, suggesting the involvement of the intrinsically disordered NTD. Three other proteins (TFII-I, PSF, and hnRNP A2B1) are also co-precipitated with Gal3 but this is not sensitive to prior nuclease treatment of the nuclear extract. The association of Gal3 with the general transcription factor TFII-I is mediated through the CRD; lactose and thiodigalactoside, saccharide ligands that bind to Gal3, inhibit the interaction while control saccharides that do not bind Gal3 fail to have any effect. These findings are of interest inasmuch as a proteomic analysis identified TFII-I as a spliceosome component and saccharide inhibition of cell-free splicing exhibited the same specificity as inhibition of the Gal3-TFII-I association.
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