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Mol. Cell. Biol., Aug 1997, 4730-4737, Vol 17, No. 8
A Vyakarnam, SF Dagher, JL Wang and RJ Patterson
Galectins are a family of beta-galactoside-binding proteins that contain
characteristic amino acid sequences in the carbohydrate recognition domain
(CRD) of the polypeptide. The polypeptide of galectin-1 contains a single
domain, the CRD. The polypeptide of galectin-3 has two domains, a
carboxyl-terminal CRD fused onto a proline- and glycine-rich amino-terminal
domain. In previous studies, we showed that galectin-3 is a required factor
in the splicing of nuclear pre-mRNA, assayed in a cell-free system. We now
document that (i) nuclear extracts derived from HeLa cells contain both
galectins-1 and -3; (ii) depletion of both galectins from the nuclear
extract either by lactose affinity adsorption or by double-antibody
adsorption results in a concomitant loss of splicing activity; (iii)
depletion of either galectin-1 or galectin-3 by specific antibody
adsorption fails to remove all of the splicing activity, and the residual
splicing activity is still saccharide inhibitable; (iv) either galectin-1
or galectin-3 alone is sufficient to reconstitute, at least partially, the
splicing activity of nuclear extracts depleted of both galectins; and (v)
although the carbohydrate recognition domain of galectin-3 (or galectin-1)
is sufficient to restore splicing activity to a galectin- depleted nuclear
extract, the concentration required for reconstitution is greater than that
of the full-length galectin-3 polypeptide. Consistent with these functional
results, double-immunofluorescence analyses show that within the nucleus,
galectin-3 colocalizes with the speckled structures observed with splicing
factor SC35. Similar results are also obtained with galectin-1, although in
this case, there are areas of galectin-1 devoid of SC35 and vice versa.
Thus, nuclear galectins exhibit functional redundancy in their splicing
activity and partition, at least partially, in the nucleoplasm with another
known splicing factor.
Copyright © 1997, American Society for Microbiology
Evidence for a role for galectin-1 in pre-mRNA splicing
Department of Biochemistry, Michigan State University, East Lansing 48824, USA.
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