Calcium-calmodulin kinase I cooperatively regulates nucleocytoplasmic shuttling of CCTα by accessing a nuclear export signal

We identified a new calmodulin kinase I (CaMKI) substrate, cytidyltransferase (CCT alpha), a crucial enzyme required for maintenance of cell membranes. CCT alpha becomes activated with translocation from the cytoplasm to the nuclear membrane, resulting in increased membrane phospholipids. Calcium-activated CCT alpha nuclear import is mediated by binding of its C-terminus to 14-3-3 zeta, a regulator of nuclear trafficking. Here CaMK1 phosphorylates residues within this C-terminus that signals association of CCT alpha with 14-3-3 zeta to initiate calcium-induced nuclear entry. CaMKI docks within the CCT alpha membrane-binding domain (residues 290-299), a sequence that displays similarities to a canonical nuclear export signal (NES) that also binds CRM1/exportin 1. Expression of a CFP-CCT alpha mutant lacking residues 290-299 in cells results in cytosolically retained enzyme. CRM1/exportin 1 was required for CCT alpha nuclear export, and its overexpression in cells was partially sufficient to trigger CCT alpha nuclear export despite calcium stimulation. An isolated CFP-290-299 peptide remained in the nucleus in the presence of leptomycin B but was able to target to the cytoplasm with farnesol. Thus CaMKI vies with CRM1/exportin 1 for access to a NES, and assembly of a CaMKI-14-3-3.-CCT alpha complex is a key effector mechanism that drives nuclear CCTa translocation.