Inhibition of Cell-Free Splicing by Saccharides That Bind Galectins and SR Proteins

The carbohydrate-binding proteins galectin-1 and galectin-3 are found in the nucleus of cells. Using a cell-free assay, depletion and reconstitution experiments have documented that these two proteins are required factors in pre-mRNA splicing. Thiodigalactoside, which binds to both galectins, inhibited the splicing reaction, whereas cellobiose, which binds neither protein, failed to yield the same effect. Although L-rhamnose does not bind to either galectin-1 or galectin-3, it does inhibit the cell-free splicing assay. The effect of rhamnose is best explained by its strong binding to Sfrs1, one member of another family of nuclear splicing factors (the SR proteins) that exhibit carbohydratebinding activity.