Dissociation of the carbohydrate-binding and splicing activities of galectin-1

Abstract

Galectin-1 (Gal1) and galectin-3 (Gal3) are two members of a family of carbohydrate-binding proteins that are found in the nucleus and that participate in pre-mRNA splicing assayed in a cell-free system. When nuclear extracts (NE) of HeLa cells were subjected to adsorption on a fusion protein containing glutathione S-transferase (GST) and Gal3, the general transcription factor II-I (TFII-I) was identified by mass spectrometry as one of the polypeptides specifically bound. Lactose and other saccharide ligands of the galectins inhibited GST-Gal3 pull-down of TFII-I while non-binding carbohydrates failed to yield the same effect. Similar results were also obtained using GST-Gal1. Site-directed mutants of Gal1, expressed and purified as GST fusion proteins, were compared with the wild-type (WT) in three assays: (a) binding to asialofetuin-Sepharose as a measure of the carbohydrate-binding activity; (b) pull-down of TFII-I from NE; and (c) reconstitution of splicing in NE depleted of galectins as a test of the in vitro splicing activity. The binding of GST-Gal1(N46D) to asialofetuin-Sepharose was less than 10% of that observed for GST-Gal1(WT), indicating that the mutant was deficient in carbohydrate-binding activity. In contrast, both GST-Gal1(WT) and GST-Gal1(N46D) were equally efficient in pull-down of TFII-I and in reconstitution of splicing activity in the galectin-depleted NE. Moreover, while the splicing activity of the wild-type protein can be inhibited by saccharide ligands, the carbohydrate-binding deficient mutant was insensitive to such inhibition. Together, all of the results suggest that the carbohydrate-binding and the splicing activities of Gal1 can be dissociated and therefore, saccharide-binding, per se, is not required for the splicing activity.