Isoform-specific differences in rapid nucleocytoplasmic shuttling cause distinct subcellular distributions of 14-3-3 sigma and 14-3-3 zeta.

Nucleocytoplasmic transport of proteins plays an important role in the regulation of many cellular processes. Differences in nucleocytoplasmic shuttling can provide a basis for isoform-specific biological functions for members of multigene families, like the 14-3-3 protein family. Many organisms contain multiple 14-3-3 isoforms, which play a role in numerous processes, including signalling, cell cycle control and apoptosis. It is still unclear whether these isoforms have specialised biological functions and whether this specialisation is based on isoform-specific ligand binding, expression regulation or specific localisation. Therefore, we studied the subcellular distribution of 14-3-3 sigma and 14-3-3 zeta in vivo in various mammalian cell types using yellow fluorescent protein fusions and isoform-specific antibodies. 14-3-3 sigma was mainly localised in the cytoplasm and only low levels were present in the nucleus, whereas 14-3-3 zeta was found at relatively higher levels in the nucleus. Fluorescence recovery after photobleaching (FRAP) experiments indicated that the 14-3-3 proteins rapidly shuttle in and out of the nucleus through active transport and that the distinct subcellular distributions of 14-3-3 sigma and 14-3-3 zeta are caused by differences in nuclear export. 14-3-3 sigma had a 1.7x higher nuclear export rate constant than 14-3-3 zeta, while import rate constants were equal. The 14-3-3 proteins are exported from the nucleus at least in part by a Crm1-dependent, leptomycin B-sensitive mechanism. The differences in subcellular distribution of 14-3-3 that we found in this study are likely to reflect a molecular basis for isoform-specific biological specialisation.