Dynamics of galectin-3 in the nucleus and cytoplasm

Abstract

This review summarizes selected studies on galectin-3 (Gal3) as an example of the dynamic behavior of a carbohydrate-binding protein in the cytoplasm and nucleus of cells. Within the 15-member galectin family of proteins, Gal3 (M(r) approximately 30,000) is the sole representative of the chimera subclass in which a proline- and glycine-rich NH(2)-terminal domain is fused onto a COOH-terminal carbohydrate recognition domain responsible for binding galactose-containing glycoconjugates. The protein shuttles between the cytoplasm and nucleus on the basis of targeting signals that are recognized by importin(s) for nuclear localization and exportin-1 (CRM1) for nuclear export. Depending on the cell type, specific experimental conditions in vitro, or tissue location, Gal3 has been reported to be exclusively cytoplasmic, predominantly nuclear, or distributed between the two compartments. The nuclear versus cytoplasmic distribution of the protein must reflect, then, some balance between nuclear import and export, as well as mechanisms of cytoplasmic anchorage or binding to a nuclear component. Indeed, a number of ligands have been reported for Gal3 in the cytoplasm and in the nucleus. Most of the ligands appear to bind Gal3, however, through protein-protein interactions rather than through protein-carbohydrate recognition. In the cytoplasm, for example, Gal3 interacts with the apoptosis repressor Bcl-2 and this interaction may be involved in Gal3's anti-apoptotic activity. In the nucleus, Gal3 is a required pre-mRNA splicing factor; the protein is incorporated into spliceosomes via its association with the U1 small nuclear ribonucleoprotein (snRNP) complex. Although the majority of these interactions occur via the carbohydrate recognition domain of Gal3 and saccharide ligands such as lactose can perturb some of these interactions, the significance of the protein's carbohydrate-binding activity, per se, remains a challenge for future investigations.

Figure 1

(A) The amino acid sequence of the carboxyl-terminal 28 amino acids residues of several homologs of the Gal3 polypeptide. The Protein Data Bank accession numbers are as follows: human (NP 002297), mouse (NP 034835), long-tailed hamster (CAA55479), Norway rat (NP 114020), rabbit (NP 001075807), dog (P38486), cattle (NP 001095811), salmon (NP 001134305), chicken (NP 999756). Sequences were retrieved and aligned by Vector NTI (Invitrogen). Residue numbers corresponding to the positions on the polypeptides of each respective species are shown at left. The conserved residues implicated in nuclear import, corresponding to R224 in the human sequence and I-LT in the murine sequence, are highlighted in boldface type, and the conserved hydrophobic-rich NES is shown with white letters on a black background. The cylinder and the arrows beneath the sequences align them with the corresponding single α helix and the S2 and F1 β strands in the crystal structure of the CRD shown in panel B. (B) Ribbon diagram showing the three-dimensional structure of the polypeptide backbone of the human CRD of Gal3 (Protein Data Bank # 2NN8 [92]), which was generated with PyMOL (W.L. DeLano, The PyMOL Molecular Graphics System, DeLano Scientific LLC, Palo Alto, CA, USA. http://www.pymol.org). The strands of the two β sheets that comprise the β sandwich are labeled F1-F5 for the five stranded sheet and S1-S6 for the six-stranded sheet. The residues determined to be critical for nuclear import are displayed with space filling representations and are numbered according to the human sequence.

Figure 2

Diagram showing the association of Gal3 with snRNPs and the pre-mRNA splicing substrate. The pre-mRNA is shown on the left-side as two rectangular exons joined by a single line intron. Newly identified associations of Gal3 with snRNPs outside the spliceosome are indicated on the right-side. Gal3 can enter the splicing pathway at an early assembly stage via its association with U1 snRNP. In addition, Gal3 also associates with multiple snRNPs in larger complexes outside of the spliceosome. Solid arrows indicate steps supported by accumulated experimental data; dashed arrows represent hypothetical Gal3 involvement that require experimental confirmation.