The evolutionarily conserved RNA-binding protein Muscleblind can function as both a splicing regulator in the nucleus and a mRNA stabilizer in the cytosol. C. elegans mbl-1/ Muscleblind undergoes alternative splicing to generate long and short isoforms that contain one or two KR motifs needed for nuclear localization. We generate three alleles that express MBL-1 proteins with two, one, or no KR motifs and find that the proteins with two KR motifs are restricted in the nucleus and could not promote neurite growth in a sensitized background. Surprisingly, proteins with one or no KR motifs are located in both cytoplasm and nucleus.
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