Our data demonstrate that galectin-3 interacts with hnRNPA2B1 in the cell nucleus and that the presence of galectin-3 modulates mRNA-export and -splicing. Although we cannot formally rule out that the galectin-3 binding to hnRNPA2B1 is indirect our ability to detect galectin-3/hnRNPA2B1 complexes in different cell types using different biochemical approaches argues that this association is robust. It had already been demonstrated that hnRNPA2B1 is part of the splicing machinery and is involved in mRNA-processing [
34,
35]. The hnRNP proteins A1, A2, B1 and B2, together with C1 and C2 assemble into particles to recruit the newly transcribed pre-mRNA into hnRNPs. They assemble into tetrameric (A2)
3(B1)- or pentameric (A2)
3(B1)(B2)-complexes in the particle-center with A1, C1 and C2 positioned peripherally [
36]. This so-called H-complex of the splicing pathway has a regulatory function and influences the ability of particular RNAs to assemble productive splicing complexes. The H-complex is the first step before the initiation complex starts to recognize the initiation sites (E- and A-complex) on the pre-mRNA. Then, the B- and C-complex initiate the two transesterification reactions leading to exon-joining and intron-release (reviewed in [
37]). Galectin-3 as interaction partner of hnRNPA2B1 in the H-complex would thus be involved in early steps of spliceosome assembly. This lectin had been already described as a factor that modulates the activity and formation of splicing complexes in HeLa cells [
17]. It was also speculated that galectin-3 binds a common splicing partner through protein-protein interactions [
38]. Furthermore, Wang et al. demonstrated that galectin-1 and galectin-3 are functionally redundant splicing factors as suggested earlier [
31], and experiments with the C-terminal carbohydrate recognition domain of galectin-3 indicate that the amino-terminal domain of the lectin modulates splicing. Carbohydrates do not seem to be involved in the interaction of galectins with spliceosomal components as previously described for galectin-1 by Voss et al. [
39]. Additional evidence for the presence of galectin-3 in spliceosomes comes from colocalisation experiments with the speckles-marker Sc35 [
31] or the Sm epitopes of the small nuclear ribonucleoprotein complexes (snRNP) [
40]. Moreover, galectin-3 sedimented in cesium sulfate gradients at densities consistent with the ones of hnRNPs and snRNPs [
16]. These observations strongly suggest that galectin-3 is a member of the spliceosomal complex. However, the role and number of galectin-3-interaction-partners in this process is unclear. According to Wang et al. the pre-mRNA as binding partner could be excluded [
38]. Haudek et al. described that about 70 % of nuclear galectin-3 is complexed in high molecular mass particles [
41]. They identified the U1-specific protein, U1 70 K, of the E-complex as galectin-3 interaction partner. We have now found additional binding partners of galectin-3 by co-immunoprecipitation and mass spectrometry, which associate at an early stage of spliceosome assembly as members of the H-, E- and A-complex, most prominently hnRNPA2B1 from the H-complex. A putative role of galectin-3 in H-complex assembly comes from another study showing that addition of the N-terminal galectin-3-domain arrested pre-mRNA splicing at a position corresponding to the H-complex [
42]. In that study, galectin-1 pulled down Gemin4. Fragments of Gemin4 also exhibited dominant negative effects when added to a cell-free splicing assay. In addition, galectin-1 co-immunoprecipitated galectin-3 from NE [
42]. Our isolation of galectin-1 from galectin-3/ sepharose columns confirmed this observation. It thus seems likely that the two galectins are members of identical splicing complexes. Another hint for this idea comes from the functional redundancy of galectin-1 and −3 as described earlier for pre-mRNA splicing [
31] and which is shown for mRNA-export by our study. RNA-splicing and nuclear export are directly linked to each other [
43] so that a reduction in RNA-export efficiency may be due to alterations in the splicing machinery and/or to the assembly of nuclear RNA-binding factors in specifying the cytoplasmic fate of an RNA molecule.
The observation, that the number or processing of RNA-transcripts for the protein SET is affected by both galectin-3- as well as by hnRNPA2B1-depletion, points to specific functions of the nuclear galetin-3/hnRNPA2B1-complex in the regulation of SET expression. Interestingly, SET expression is deregulated in more than 10 % of kidney cancer samples [
44], a cancer type where we have previously demonstrated increased nuclear translocation of galectin-3 [
18]. SET is a key modulator in cell proliferation and interacts with hnRNPA2B1 [
45]. Moreover, SET and hnRNPA2B1 are specific inhibitors of the tumor suppressor protein phosphatase 2A (PPA2), a major phosphatase that controls cell proliferation [
45,
46]. Consequently, our results suggest that in addition to PPA2-inhibition, hnRNPA2B1 in complex with galectin-3 stimulates cell proliferation by increasing the number of protein coding SET-transcripts.