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Nuclear Localization of Endogenous RGK Proteins and Modulation of Cell Shape Remodeling by Regulated Nuclear Transport
Author(s) -
Mahalakshmi Ramasubbu N.,
Ng Mei Yong,
Guo Ke,
Qi Zeng,
Hunziker Walter,
Béguin Pascal
Publication year - 2007
Publication title -
traffic
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.677
H-Index - 130
eISSN - 1600-0854
pISSN - 1398-9219
DOI - 10.1111/j.1600-0854.2007.00599.x
Subject(s) - nuclear transport , importin , nuclear localization sequence , nls , microbiology and biotechnology , biology , phosphorylation , subcellular localization , nuclear protein , ran , cytosol , cell nucleus , transport protein , cytoplasm , transcription factor , biochemistry , gene , enzyme
The members of the RGK small GTP‐binding protein family, Kir/Gem, Rad, Rem and Rem2, are multifunctional proteins that regulate voltage‐gated calcium channel activity and cell shape remodeling. Calmodulin (CaM) or CaM 14‐3‐3 are regulators of RGK functions and their association defines the subcellular localization of RGK proteins. Abolition of CaM association results in the accumulation of RGK proteins in the nucleus, whereas 14‐3‐3 binding maintains them in the cytoplasm. Kir/Gem possesses nuclear localization signals (NLS) that mediate nuclear accumulation through an importin α5‐dependent pathway (see Mahalakshmi RN, Nagashima K, Ng MY, Inagaki N, Hunziker W, Béguin P. Nuclear transport of Kir/Gem requires specific signals and importin α5 and is regulated by Calmodulin and predicted service phosphorylations. Traffic 2007; doi: 10.1111/j.1600‐0854.2007.00598.x). Because the extent of nuclear localization depends on the RGK protein and the cell type, the mechanism and regulation of nuclear transport may differ. Here, we extend our analysis to the other RGK members and show that Rem also binds importin α5, whereas Rad associates with importins α3, α5 and β through three conserved NLS. Predicted phosphorylation of a serine residue within the bipartite NLS affects, as observed for Kir/Gem, nuclear accumulation of Rem, but not that of Rad or Rem2. We also identify an additional regulatory phosphorylation for all RGK proteins that prevents binding of 14‐3‐3 and thereby interferes with their cytosolic relocalization by 14‐3‐3. Functionally, nuclear localization of RGK proteins contributes to the suppression of RGK‐mediated cell shape remodeling. Importantly, we show that endogenous RGK proteins are localized predominantly in the nucleus of individual cells of the brain cortex ‘ in situ ’ as well as in primary hippocampal cells, indicating that transport between the nucleus and their site of action in the cytoplasm (i.e., cytoskeleton, endoplasmic reticulum or plasma membrane) is of physiological relevance for the regulation of RGK protein function.