Isoform-specific differences in rapid nucleocytoplasmic shuttling cause distinct subcellular distributions of 14-3-3σ and 14-3-3ζ

Abstract

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 14-3-3σ and 14-3-3ζ in vivo in various mammalian cell types using yellow fluorescent protein fusions and isoform-specific antibodies. 14-3-3σ was mainly localised in the cytoplasm and only low levels were present in the nucleus, whereas 14-3-3ζ 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 14-3-3σ and 14-3-3ζ are caused by differences in nuclear export. 14-3-3σ had a 1.7× higher nuclear export rate constant than 14-3-3ζ, 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.

Clinical Significance of 14-3-3 Zeta in Human Esophageal Cancer

“…We found 14-3-3 zeta expression to be predominantly cytoplasmic (93%), though substantial (63%) nuclear expression was also observed. Our observation is in line with the work of Van Hermert et al (25), which reported most of 14-3-3 zeta to be cytosolically located and a significant amount to be present in the nuclei of various mammalian cells. Overexpression of 14-3-3 zeta in cytoplasm and nucleus may suggest its role both in evading apoptosis and in cell proliferation.…”

Proteomic analysis identifies that 14-3-3ζ interacts with β-catenin and facilitates its activation by Akt

“…While this manuscript was in preparation, Lo and colleagues (32) reported that in Caenorhabditis elegans 14-3-3͞PAR-5 binds to and mediates nuclear export of TCF͞POP1. It is not clear which 14-3-3 isoform is the mammalian orthologue responsible for binding to TCF, because different isoforms of 14-3-3 afford different rates of nucleocytoplasmic transport (33); nonetheless, the critical role of 14-3-3 proteins in the ␤-catenin͞TCF pathway is evident.…”

14-3-3 proteins—an update

“…In addition, significant structural differences between the 14-3-3 isoforms have been detected and these differences seem to reside adjacent to the amphipathic groove, a region where 14-3-3 binds with numerous phsophoserine ligands. Study by van Hermet et al showed a difference in the protein distribution among the 14-3-3 isoforms [71]. They found that 14-3-3σ is mainly localized in the cytoplasm and only low levels are present in the nucleus.…”