Understanding the biochemical activities of galectin-1 and galectin-3 in the nucleus.

Nuclear extracts (NE), capable of carrying out splicing of pre-mRNA, contain galectin-1 and galectin-3. NE depleted of galectins-1 and -3 concomitantly lose their splicing activity. The activity of the galectin-depleted extract can be reconstituted by the addition of either galectin-1 or galectin-3. These results suggest that galectins-1 and -3 serve as redundant splicing factors. Consistent with this notion, immunofluorescence staining showed that both galectins yielded a diffuse nucleoplasmic distribution, matching that of nascent transcripts and consistent with the hypothesis that bulk transcription and pre-mRNA processing occur throughout the nucleoplasm. Under some conditions, the galectins could be found in speckled structures and nuclear bodies but the prevailing thought is that these represent sites of storage and recycling rather than sites of action. Galectin-1 and galectin-3 bind directly to Gemin4, a component of the SMN core complex, which plays multiple roles in ribonucleoprotein assembly, including the biogenesis, delivery, and recycling of snRNPs to the spliceosome. Thus, galectin-1 and galectin-3 constitute a part of an interacting dynamic network of many factors involved in the splicing and transport of mRNA.