Identification of galectin-3 as a factor in pre-mRNA splicing.

Abstract

Galectin-3 (Mr 35,000) is a galactose/lactose-specific lectin found in association with ribonucleoprotein complexes in many animal cells. Cell-free-splicing assays have been carried out to study the requirement for galectin-3 in RNA processing by HeLa cell nuclear extracts by using 32P-labeled MINX as the pre-mRNA substrate. Addition of saccharides that bind galectin-3 with high affinity inhibited product formation in the splicing assay, while addition of carbohydrates that do not bind to the lectin did not inhibit product formation. Nuclear extracts depleted ofgalectin-3 by affinity adsorption on a lactoseagarose column were deficient in splicing activity. Extracts subjected to parallel adsorption on control celiobiose-agarose retained splicing activity. The activity of the galectin-3-depleted extract could be reconstituted by the addition of purified recombinant galectin-3, whereas the addition of other lectins, either with a similar saccharide binding specificity (soybean agglutinin) or with a different specificity (wheat germ agglutinin), did not restore splicing activity. The formation of splicing complexes was also sensitive to galectin-3 depletion and reconstitution. Together, these results define a requirement for galectin-3 in pre-mRNA splicing and identify it as a splicing factor.Galectin-3 (1) is the name for the galactose(Gal)/lactose (Lac)-specific lectin previously known under a number of different designations, including carbohydrate binding protein 35 (CBP35) (2), Mac-2 (3), IgE-binding protein (4), CBP30 (5), L-29 (6), and L-34 (7). In this communication, we will use the designation galectin-3 when we refer to the gene or protein in general, assuming that studies carried out on the gene or protein under any one of the above names is applicable to all of them. There are instances, however, in which the specific molecule used by one laboratory is slightly (but significantly) different from the corresponding molecule of another laboratory-e.g., the cDNAs reported for murine CBP35, Mac-2, and L-34 are of different lengths. In this case, we will use the old designation to highlight the specific source of the molecule.The galectin family of animal lectins is distinguished by the Gal/Lac specificity of its carbohydrate recognition domain (CRD), with highly conserved residues between members of the family (galectin-1, -2, -3, and -4) and between the homologs found in various species for any given single member (for reviews see refs. 8 and 9). The polypeptide of galectin-3 is delineated into two domains (2,8): an N-terminal half that is proline and glycine rich, with limited similarity to proteins of the heterogeneous nuclear ribonucleoprotein (RNP) complexes, and a C-terminal half that is similar to the CRD of other members of the galectin family. The majority of galectin-3 is found in the cytoplasm and nucleus of mouse 3T3 fibroblasts in the form of RNP complexes (10, 11). For example, treatment of permeabilized cells with ribonuclease A released the lectin from the nucleus with concomitant los...

Identification of mRNA splicing factors as the endothelial receptor for carbohydrate-dependent lung colonization of cancer cells

“…Identification of Sfrs proteins as carbohydrate-binding proteins was unexpected, but this finding parallels observations on galectins and annexins. Galectins are well-established carbohydrate-binding proteins (22), but several galectin proteins also localize to the nucleus and exhibit pre-mRNA splicing activity (23)(24)(25). The annexins also form a large protein family, and some members localize to the nucleus and bind RNA (25)(26)(27).…”

MicroRNA-424 Predicts a Role for β-1,4 Branched Glycosylation in Cell Cycle Progression

“…MCF-7-MGAT4A-KD cells showed a pronounced increase in nuclear galectin-3 by fluorescence microscopy. Nuclear galectin-3 plays a range of biological roles, including manipulation of RNA splicing and enhancement of transcription factor activity (46,47). Nuclear galectin-3 also plays a role in late-G 1 cell cycle arrest and suppression of anoikis, the programmed cell death in response to loss of cell adhesion (34,35).…”

Clinical significance of circulating galectins as colorectal cancer markers

“…to date, 15 mammalian galectins have been identified and characterized. They are expressed both inside and outside cells and play important roles in a variety of cellular biological processes (8)(9)(10)(11), including various stages of tumor progression and/or malignancy (12,13). galectin-1, galectin-3, and galectin-4 appear to be involved in the early development of human colon carcinoma (14) and the malignant transformation of the tumor (15).…”