Loss of SMEK, a Novel, Conserved Protein, Suppresses mek1 Null Cell Polarity, Chemotaxis, and Gene Expression Defects

Abstract

MEK/extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase signaling is imperative for proper chemotaxis. Dictyostelium mek1؊ (MEK1 null) and erk1 ؊ cells exhibit severe defects in cell polarization and directional movement, but the molecules responsible for the mek1 ؊ and erk1 ؊ chemotaxis defects are unknown. Here, we describe a novel, evolutionarily conserved gene and protein (smkA and SMEK, respectively), whose loss partially suppresses the mek1 ؊ chemotaxis phenotypes. SMEK also has MEK1-independent functions: SMEK, but not MEK1, is required for proper cytokinesis during vegetative growth, timely exit from the mound stage during development, and myosin II assembly. SMEK localizes to the cell cortex through an EVH1 domain at its N terminus during vegetative growth. At the onset of development, SMEK translocates to the nucleus via a nuclear localization signal (NLS) at its C terminus. The importance of SMEK's nuclear localization is demonstrated by our findings that a mutant lacking the EVH1 domain complements SMEK deficiency, whereas a mutant lacking the NLS does not. Microarray analysis reveals that some genes are precociously expressed in mek1 ؊ and erk1 ؊ cells. The misexpression of some of these genes is suppressed in the smkA deletion. These data suggest that loss of MEK1/ERK1 signaling compromises gene expression and chemotaxis in a SMEK-dependent manner.Chemotaxis is a conserved cellular process in which cells detect a chemical gradient, polarize, and proceed up the gradient by extending a pseudopod at their leading edge and retracting their posteriors (17,27,40). In both mammalian and Dictyostelium cells, mitogen-activated protein (MAP) kinase pathways are required for proper chemotaxis. Mouse knockout studies have shown that MEKK1, a MAP kinase kinase kinase that scaffolds and activates both the extracellular signal-regulated kinase (ERK) ERK1/2 and stress-activated Jun N-terminal protein kinase (JNK) MAP kinase pathways, is required for epithelial and fibroblast migration (65). Fibroblasts from mek1 Ϫ/Ϫ (MEK1 is a MAP kinase kinase that specifically activates ERK1/2) and jnk1 Ϫ/Ϫ jnk2 Ϫ/Ϫ mice also exhibit reduced directional motility in fibronectin and wound-induced migration assays (20,25). In addition, ERK1/2 activation is required for MDCK epithelial cells moving as a sheet or undergoing a partial epithelial-to-mesenchymal transition during tubulogenesis (36, 39).One MEK and two ERK family MAP kinases have been identified in Dictyostelium. Cells lacking MEK1 or ERK1 (mek1 Ϫ and erk1 Ϫ cells) exhibit a marked absence of polarity and severely reduced chemotaxis speed and directionality (34,50). Dictyostelium MEK1 is required for ERK1 activation and its localization to the cell cortex in response to chemoattractant stimulation (50). ERK2 is not regulated by MEK1 but has an established role in phosphorylating the RegA phosphodiesterase during starvation-induced cyclic AMP (cAMP) relay signaling (35).Although the ERK and JNK MAP kinase pathways are required for cell motility in...

Identification of SMEK2 as a candidate gene for regulation of responsiveness to dietary cholesterol in rats

“…This finding implies that MEK/ERK signal transduction is associated with TG levels in the liver and regulates VLDL components (15). SMEK2 was originally identified by Mendoza et al (16) as a suppressor of MEK1 in Dictyostelium using a mek1 mutant by using a second-site suppressor. They described that SMEK regulated some MEK1 effectors in a manner opposite to that of the MEK1/ERK1 pathway, and that SMEK signaling converged on a partially overlapping set of MEK1 effectors (16).…”

“…Thus, the ExHC-specific 10 bp deletion in SMEK2 is consistent with the highly decreased mRNA expression of SMEK2 in ExHC rats. In vegetative cells, SMEK localizes to the cell cortex through the EVH1 domain and translocates to the nucleus via a nuclear localization signal at its C terminus basic stretch in response to starvation signals (16). Because the SMEK2 protein lacks the conserved C-terminal basic stretch in ExHC rats (Fig.…”

“…SMEK2 was originally identified by Mendoza et al (16) as a suppressor of MEK1 in Dictyostelium using a mek1 mutant by using a second-site suppressor. They described that SMEK regulated some MEK1 effectors in a manner opposite to that of the MEK1/ERK1 pathway, and that SMEK signaling converged on a partially overlapping set of MEK1 effectors (16). These results suggest that SMEK is associated with the regulation of liver TG levels in a manner opposite to that of the MEK/ERK pathway.…”

Smek promotes histone deacetylation to suppress transcription of Wnt target gene brachyury in pluripotent embryonic stem cells

“…Smek functions in a variety of molecular processes, including regulation of MEK in Dictyostelium cells [10], dephosphorylation of γ-H2AX in somatic mammalian cells [9], transcriptional modulation of DAF-16/FOXO3a in C. elegans [11], and regulation of the intracellular localization of Miranda in Drosophila neuroblasts [12]. However, the role of Smek in molecular and cellular processes in ESCs is unknown.…”

Psy2 Targets the PP4 Family Phosphatase Pph3 To Dephosphorylate Mth1 and Repress Glucose Transporter Gene Expression

“…The highly conserved R3/Psy2 regulatory subunit has been implicated in nutrient starvation-induced chemotaxis in Dictyostelium, where it acts as a suppressor for the mitogen-activated protein kinase kinase MEK1 and hence is known as SMEK (17). In the present study, we uncovered a novel function of Psy2 in the context of nutrient sensing in Saccharomyces cerevisiae.…”