IDENTIFICATION OF GALECTIN-3 AS A FACTOR IN PRE-MESSENGER-RNA SPLICING

Galectin-3 (M(r) approximate to 35,000) is a galactose/lactose-specifi c lectin found in association with ribonucleoprotein complexes in many animal cells. Cell-free-splicing assays have been carried out to stud y the requirement for galectin-3 in RNA processing by HeLa cell nuclea r extracts by using P-32-labeled MINX as the pre-mRNA substrate. Addit ion of saccharides that bind galectin-3 with high affinity inhibited p roduct formation in the splicing assay, while addition of carbohydrate s that do not bind to the lectin did not inhibit product formation. Nu clear extracts depleted of galectin-3 by affinity adsorption on a lact ose-agarose column were deficient in splicing activity. Extracts subje cted to parallel adsorption on control cellobiose-agarose retained spl icing activity. The activity of the galectin-3-depleted extract could be reconstituted by the addition of purified recombinant galectin-3, w hereas the addition of other lectins, either with a similar saccharide binding specificity (soybean agglutinin) or with a different specific ity (wheat germ agglutinin), did not restore splicing activity. The fo rmation of splicing complexes was also sensitive to galectin-3 depleti on and reconstitution, Together, these results define a requirement fo r galectin-3 in pre-mRNA splicing and identify it as a splicing factor .