Cofilin mediates lamellipodium extension and polarized cell migration by stimulating actin filament dynamics at the leading edge of migrating cells. Cofilin is inactivated by phosphorylation at Ser-3 and reactivated by cofilin-phosphatase Slingshot-1L (SSH1L). Little is known of signaling mechanisms of cofilin activation and how this activation is spatially regulated. Here, we show that cofilin-phosphatase activity of SSH1L increases ∼10-fold by association with actin filaments, which indicates that actin assembly at the leading edge per se triggers local activation of SSH1L and thereby stimulates cofilin-mediated actin turnover in lamellipodia. We also provide evidence that 14-3-3 proteins inhibit SSH1L activity, dependent on the phosphorylation of Ser-937 and Ser-978 of SSH1L. Stimulation of cells with neuregulin-1β induced Ser-978 dephosphorylation, translocation of SSH1L onto F-actin–rich lamellipodia, and cofilin dephosphorylation. These findings suggest that SSH1L is locally activated by translocation to and association with F-actin in lamellipodia in response to neuregulin-1β and 14-3-3 proteins negatively regulate SSH1L activity by sequestering it in the cytoplasm.

Activating transcription factor-2 (ATF-2) is a basic region leucine zipper protein whose DNA target sequence is the widely distributed cAMP response element (CRE). We report here that mice carrying a germline mutation in ATF-2 demonstrated unique actions of ATF-2 not duplicated by other ATF/CREB family members. Mutant mice had decreased postnatal viability and growth, with a defect in endochondral ossification at epiphyseal plates similar to human hypochondroplasia. The animals had ataxic gait, hyperactivity and decreased hearing. In the brain, there were reduced numbers of cerebellar Purkinje cells, atrophic vestibular sense organs and enlarged ventricles. Unlike CREB alpha/delta-deficient mice whose main defect is in long-term potentiation, the widespread abnormalities in ATF-2 mutant mice demonstrate its absolute requirement for skeletal and central nervous system development, and for maximal induction of select genes with CRE sites, such as E-selectin.

We previously identified testicular protein kinase 1 (TESK1), which phosphorylates cofilin and induces actin cytoskeletal reorganization. We now report identification and characterization of another member of a TESK family, testicular protein kinase 2 (TESK2), with 48% amino acid identity with TESK1. Like TESK1, TESK2 phosphorylated cofilin specifically at Ser-3 and induced formation of actin stress fibers and focal adhesions. Both TESK1 and TESK2 are highly expressed in the testis, but in contrast to TESK1, which is predominantly expressed in testicular germ cells, TESK2 is expressed predominantly in nongerminal Sertoli cells. Thus, TESK1 and TESK2 seem to play distinct roles in spermatogenesis. In HeLa cells, TESK1 was localized mainly in the cytoplasm, whereas TESK2 was localized mainly in the nucleus, which means that TESK1 and TESK2 likely have distinct cellular functions. Because the kinase-inactive mutant of TESK2 was localized in the cytoplasm, nuclear/cytoplasmic localization of TESK2 depends on its kinase activity. A TESK2 mutant lacking the C-terminal noncatalytic region had about a 10-fold higher kinase activity in vitro and, when expressed in HeLa cells, induced punctate actin aggregates in the cytoplasm and unusual condensation and fragmentation of nuclei, followed by apoptosis. Thus, we propose that the C-terminal region plays important roles in regulating the kinase activity and cellular functions of TESK2.The dynamics of polymerization/depolymerization of actin filaments and their remodeling are essential for cell movement, adhesion, and division (1). Cofilin and actin-depolymerizing factor (ADF) 1 play an essential role in the rapid turnover of actin filaments and actin-based cytoskeletal reorganization by stimulating depolymerization and severance of actin filaments (2-4). As the activity of cofilin/ADF is negatively regulated by phosphorylation at Ser-3 (5), enzymes phosphorylating cofilin/ ADF seem to play important roles in actin filament dynamics. We and other investigators provide evidence that LIM kinase 1 (LIMK1) and LIM kinase 2 (LIMK2) (6 -8) phosphorylate cofilin/ADF specifically at Ser-3 and induce actin cytoskeletal reorganization by phosphorylating and inactivating cofilin (9, 10). LIM kinases are activated in cultured cells by Rho family small GTPases Rac, Rho, and Cdc42 (9 -11), this activation mediated by downstream effectors p21-activated kinase (PAK) and Rho-associated kinase, by phosphorylation of Thr-508 of LIMK1 or Thr-505 of LIMK2 (12-16).Testicular protein kinase 1 (TESK1) is a serine/threonine kinase with a structure composed of an N-terminal protein kinase domain and a C-terminal proline-rich region (17). The kinase domain of TESK1 is closely related to those of LIM kinases (17). We recently obtained evidence that TESK1, like LIM kinases, has the potential to phosphorylate cofilin/ADF specifically at Ser-3 and induces the formation of actin stress fibers and focal adhesions by phosphorylating cofilin/ADF (18). In contrast to LIM kinases, the kinase activity of TESK1 is...

The effect of silicon on organ growth and its mechanisms of action were studied in rice ( Oryza sativa L. cv. Koshihikari), oat ( Avena sativa L. cv. Victory), and wheat ( Triticum aestivum L. cv. Daichino-Minori) seedlings grown in the dark. Applying silicon in the form of silicic acid to these seedlings via culture solution resulted in growth promotion of third (rice) or second (oat and wheat) leaves. The optimal concentration of silicon was 5-10 mM. No growth promotion was observed in early organs, such as coleoptiles or first leaves. In silicon-treated rice third leaves, the epidermal cell length increased, especially in the basal regions, without any effect on the number of cells, showing that silicon promoted cell elongation but not cell division. Silicon also increased the cell wall extensibility significantly in the basal regions of rice third leaves. These results indicate that silicon stimulates growth of rice and some other Poaceae leaves by increasing cell wall extensibility.