Galectins are a family of ␤-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.Galectins are a family of widely distributed proteins that (i) bind to ␤-galactosides and (ii) contain characteristic amino acid sequences in the carbohydrate recognition domain (CRD) of the polypeptide. At present, eight mammalian galectins have been reported and classified into three subgroups, according to the content and organization of the domains in their respective polypeptides (for reviews, see references 3 and 21). The prototype subgroup consists of polypeptides (ϳ14 kDa) with a single domain, the CRD. Galectins-1, -2, -5, and -7 are members of this subgroup. Another subgroup is the tandem repeat type, which has three members: galectins-4, -6, and -8. These galectins have two domains, each a CRD, connected by a linker region. Finally, the chimera subgroup is, at present, represented by a single member, galectin-3. Its polypeptide contains two domains, a CRD fused onto a Pro-, Gly-rich domain.In previous studies, we reported the localization of galectin-3 to the cell nucleus in the form of a ribonucleoprotein (RNP) complex (23, 29). We also identified it as one of the many proteins required for the splicing of pre-mRNA, ...

No abstract

No abstract