Phosphorylation-dependent Regulation of Unique Nuclear and Nucleolar Localization Signals of LIM Kinase 2 in Endothelial Cells

Abstract

LIM kinases (LIMKs) regulate actin dynamics through cofilin phosphorylation and also have a function in the nucleus. Recently we have shown that LIMK2 shuttles between cytoplasm and nucleus in endothelial cells and that nuclear import is inhibited by protein kinase C-mediated phosphorylation of Ser-283. Here we aimed to identify the structural features of LIMK2 responsible for nuclear import. We found that the kinase domain of LIMK2 is localized exclusively in the nucleus and, in contrast to the kinase domain of LIMK1, it accumulated in the nucleolus. Through site-directed mutagenesis, we identified the basic amino acid-rich motif KKRTLRKNDRKKR (amino acids 491-503) as the functional nuclear and nucleolar localization signal of LIMK2. After fusing this motif to enhanced green fluorescent protein, the fusion protein localized exclusively in the nucleus and nucleolus. Mutagenesis studies showed that phosphorylation of Thr-494, a putative protein kinase C phosphorylation site identified within the nuclear localization signal, inhibits nuclear import of the enhanced green fluorescent protein-PDZ kinase domain of LIMK2. After inhibiting nuclear export with leptomycin B, phosphorylation of either Ser-283 or Thr-494 reduced the nuclear import of LIMK2. Phosphorylation of both Ser-283 and Thr-494 sites inhibited nuclear import completely. Our findings identify a unique basic amino acid-rich motif (amino acids 491-503) in LIMK2 which is not present in LIMK1 that serves to target the protein not only to the nucleus but also to the nucleolus. Phosphorylation of Thr-494 within this motif negatively regulates nuclear import of LIMK2.Endothelial cell structure and functional integrity are important in the maintenance of the vessel wall and circulatory function. Contraction, migration, and proliferation of vascular endothelial cells control vascular permeability, endothelial repair after injury, and angiogenesis (1, 2). The LIM kinases (LIMKs), 2 consisting of LIMK1 and LIMK2, are serine/threonine protein kinases that regulate the actin dynamics via phosphorylating the actin-depolymerizing protein cofilin (3, 4). Besides this, various studies showed that LIMKs may have a function in the nucleus (5, 6). The phenotype of LIMK2 knock-out mouse showed a defect in spermatogenesis, suggesting a nuclear function of tLIMK2, a testis-specific LIMK2 splice form lacking both LIM domains and being preferentially localized in the nucleus (7). It has been shown that the nuclear localization of LIMKs can mediate suppression of Rac/Cdc42-mediated cyclin D1 expression. This effect of LIMKs was independent of cofilin phosphorylation and the regulation of actin dynamics (8).LIMK1 and LIMK2 are localized predominantly in the cytoplasm but accumulate in the nucleus when the cells are treated with the chromosomal region maintenance 1-dependent export inhibitor leptomycin B (LMB), suggesting that these kinases contain nuclear localization signals (NLS) (9, 10). NLSs are often characterized by clusters of basic amino acids. The main types of NLS know...

LIMK2 is a crucial regulator and effector of Aurora-A-kinase-mediated malignancy

“…Previously, PKC has been shown to phosphorylate LIMK2 at S283 and T494 sites, which reduces its nuclear import (Goyal et al, 2006). We also observed nuclear localization of LIMK2 upon Aurora A inhibition (Fig.…”

Nischarin Inhibits LIM Kinase To Regulate Cofilin Phosphorylation and Cell Invasion

“…The LIMK family consists of LIMK1 and LIMK2, both of which are serine/threonine kinases that regulate actin dynamics by phosphorylating cofilin. LIMK1 and LIMK2 are predominantly cytoplasmic (24). The major difference between LIMK1 and LIMK2 is that LIMK2 contains a second basic amino acid-rich motif between the PDZ and kinase domains, in addition to the basic amino acid-rich region in the kinase domain (24).…”

P2Y2 receptors regulate osteoblast mechanosensitivity during fluid flow

“…Phosphorylation of cofilin can occur through activation of the LIM kinase family, which consists of LIMK-1 and LIMK-2, by phosphorylation at Thr508 or Thy505, respectively (26,30,53). LIMK-1 and LIMK-2 are closely related serine-threonine kinases, whose structures contain two NH 2 -terminal LIM domains, an internal PDZ domain, and a COOH-terminal protein kinase domain (1,16,50). However, LIMK-2 is found more readily in embryonic and adult tissue compared with LIMK-1 (51).…”

“…Activated LIMK-2 mediates actin cytoskeleton reorganization in various cell types by phosphorylating, thereby inactivating, the actin-severing protein, cofilin (16,37,52,53). Cofilin binding to actin promotes depolymerization of the actin filaments while phosphorylation of this protein enables actin polymerization and stress fiber formation (16,37,53). Overall, evidence suggests that the RhoA GTPase-activated ROCK/ LIMK/cofilin pathway can regulate the actin cytoskeletal dynamics via a P2Y receptor in osteoblasts during mechanotransduction.…”

“…Upon RhoA GTPase activation, the downstream Rho kinase (ROCK) has been shown to phosphorylate LIM kinase-2 (LIMK-2) at Thr505 (1,32,49). Activated LIMK-2 mediates actin cytoskeleton reorganization in various cell types by phosphorylating, thereby inactivating, the actin-severing protein, cofilin (16,37,52,53). Cofilin binding to actin promotes depolymerization of the actin filaments while phosphorylation of this protein enables actin polymerization and stress fiber formation (16,37,53).…”