Evidence for a role for galectin-1 in pre-mRNA splicing

Galectins are a family of beta-galactoside-binding proteins that contain characteristic amino acid sequences in the carbohydrate recognition domain (CRD) of the polypeptide. The polypeptide of galectin-1 contains a single domain, the CRD. The polypeptide of galectin-3 has two domains, a carboxyl-terminal CRD fused onto a proline- and glycine-rich amino-terminal domain. In previous studies, we showed that galectin-3 is a required factor in the splicing of nuclear pre-mRNA, assayed in a cell-free system. We now document that (i) nuclear extracts derived from HeLa cells contain both galectins-1 and -3; (ii) depletion of both galectins from the nuclear extract either by lactose affinity adsorption or by double-antibody adsorption results in a concomitant loss of splicing activity; (iii) depletion of either galectin-1 or galectin-3 by specific antibody adsorption fails to remove all of the splicing activity, and the residual splicing activity is still saccharide inhibitable; (iv) either galectin-1 or galectin-3 alone is sufficient to reconstitute, at least partially, the splicing activity of nuclear extracts depleted of both galectins; and (v) although the carbohydrate recognition domain of galectin-3 (or galectin-1) is sufficient to restore splicing activity to a galectin-depleted nuclear extract, the concentration required for reconstitution is greater than that of the full-length galectin-3 polypeptide. Consistent with these functional results, double-immunofluorescence analyses show that within the nucleus, galectin-3 colocalizes with the speckled structures observed with splicing factor SC35. Similar results are also obtained with galectin-1, although in this case, there are areas of galectin-1 devoid of SC35 and vice versa. Thus, nuclear galectins exhibit functional redundancy in their splicing activity and partition, at least partially, in the nucleoplasm with another known splicing factor.