Involvement of 14-3-3 proteins in nuclear localization of telomerase

Maintenance of telomeres is implicated in chromosome stabilization and cell immortalization. Telomerase, which catalyzes de novo synthesis of telomeres, is activated in germ cells and most cancers. Telomerase activity is regulated by gene expression for its catalytic subunit, TERT, whereas several lines of evidence have suggested a post-translational regulation of telomerase activity. Here we identify the 14-3-3 signaling proteins as human TERT (hTERT)-binding partners. A dominant-negative 14-3-3 redistributed hTERT, which was normally predominant in the nucleus, into the cytoplasm. Consistent with this observation, hTERT-3A, a mutant that could not bind 14-3-3, was localized into the cytoplasm. Leptomycin B, an inhibitor of CRM1/exportin 1-mediated nuclear export, or disruption of a nuclear export signal (NES)-like motif located just upstream of the 14-3-3 binding site in hTERT impaired the cytoplasmic localization of hTERT. Compared with wild-type hTERT, hTERT-3A increased its association with CRM1. 14-3-3 binding was not required for telomerase activity either in vitro or in cell extracts. These observations suggest that 14-3-3 enhances nuclear localization of TERT by inhibiting the CRM1 binding to the TERT NES-like motif.     

The cyclin-dependent kinase 11 interacts with 14-3-3 proteins

Cyclin-dependent kinase 11 isoforms (CDK11) are members of the p34(cdc2) superfamily. They have been shown to play a role in RNA processing and apoptosis. In the present study, we investigate whether CDK11 interacts with 14-3-3 proteins. Our study shows that the putative 14-3-3 binding site (113-RHRSHS-118) within the N-terminal domain of CDK11(p110) is functional. Endogenous CDK11(p110) binds directly to 14-3-3 proteins and phosphorylation of the serine 118 within the RHRSHS motif seems to be required for the binding. Besides, CDK11(p110) is capable of interacting with several different isoforms of 14-3-3 proteins both in vitro and in vivo. The interaction of 14-3-3 gamma with CDK11(p110) occurs throughout the entire cell cycle and reaches maximum at the G2/M phase. Interestingly, 14-3-3 gamma shows strong interaction with N-terminal portion of caspase-cleaved CDK11(p110) (CDK11(p60)) product at 48 h after Fas treatment, which correlates with the maximal cleavage level of CDK11(p110) and the maximum activation level of CDK11 kinase activity during apoptosis. Collectively, these results suggest that CDK11 kinases could be regulated by interaction with 14-3-3 proteins during cell cycle and apoptosis.