Numerous physiological and pathological stimuli promote the rearrangement of the actin cytoskeleton, thereby modulating cellular motile functions. Although it seems intuitively obvious that cell motility requires coordinated protein biosynthesis, until recently the linkage between cytoskeletal actin dynamics and correlated gene activities remained unknown. This knowledge gap was filled in part by the discovery that globular actin polymerization liberates myocardin-related transcription factor (MRTF) cofactors, thereby inducing the nuclear transcription factor serum response factor (SRF) to modulate the expression of genes encoding structural and regulatory effectors of actin dynamics. This insight stimulated research to better understand the actin–MRTF–SRF circuit and to identify alternative mechanisms that link cytoskeletal dynamics and genome activity.

Smooth muscle cells switch between differentiated and proliferative phenotypes in response to extracellular cues, but the transcriptional mechanisms that confer such phenotypic plasticity remain unclear. Serum response factor (SRF) activates genes involved in smooth muscle differentiation and proliferation by recruiting muscle-restricted cofactors, such as the transcriptional coactivator myocardin, and ternary complex factors (TCFs) of the ETS-domain family, respectively. Here we show that growth signals repress smooth muscle genes by triggering the displacement of myocardin from SRF by Elk-1, a TCF that acts as a myogenic repressor. The opposing influences of myocardin and Elk-1 on smooth muscle gene expression are mediated by structurally related SRF-binding motifs that compete for a common docking site on SRF. A mutant smooth muscle promoter, retaining responsiveness to myocardin and SRF but defective in TCF binding, directs ectopic transcription in the embryonic heart, demonstrating a role for TCFs in suppression of smooth muscle gene expression in vivo. We conclude that growth and developmental signals modulate smooth muscle gene expression by regulating the association of SRF with antagonistic cofactors.

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Myocardin is a SAP (SAF-A͞B, Acinus, PIAS) domain transcription factor that associates with serum response factor (SRF) to potently enhance SRF-dependent transcription. Here we describe two myocardin-related transcription factors (MRTFs), A and B, that also interact with SRF and stimulate its transcriptional activity. Whereas myocardin is expressed specifically in cardiac and smooth muscle cells, MRTF-A and -B are expressed in numerous embryonic and adult tissues. In SRF-deficient embryonic stem cells, myocardin and MRTFs are unable to activate SRF-dependent reporter genes, confirming their dependence on SRF. Myocardin and MRTFs comprise a previously uncharacterized family of SRF cofactors with the potential to modulate SRF target genes in a wide range of tissues.

Ternary complex factors (TCFs), one of which is Elk‐1, have been implicated in mediation of c‐fos induction. They have been shown to be phosphorylated by mitogen‐activated protein kinases (MAPKs) in vitro. We demonstrate that recombinant Elk‐1 is hyperphosphorylated in vivo upon joint overexpression of MAPKs and constitutively activated Raf‐1 kinase, the latter serving as an indirect in vivo activator of MAPKs. This phosphorylation is accompanied by a conformational change and results in an elevated transactivation potential of Elk‐1. Mutation of mapped in vivo phosphorylation sites, which are potential targets for MAPKs, reduced Elk‐1‐mediated transcription. Thus, MAPKs are very probably controlling Elk‐1 activity by direct phosphorylation in vivo. Furthermore, Elk‐1 was shown to stimulate transcription from both the c‐fos serum response element and also from an Ets binding site. While binding of TCFs to the c‐fos promoter is dependent on the serum response factor, TCFs can autonomously interact with Ets binding sites. This indicates that TCFs may participate in the transcriptional regulation of two different sets of genes.

WD-repeat proteins are regulatory beta-propeller platforms that enable the assembly of multiprotein complexes. Here, we report the functional and bioinformatic analysis of human WD-repeat protein Interacting with PhosphoInosides (WIPI)-1a (WIPI49/Atg18), a member of a novel WD-repeat protein family with autophagic capacity in Saccharomyces cerevisiae and Caenorhabditis elegans, recently identified as phospholipid-binding effectors. Our phylogenetic analysis divides the WIPI protein family into two paralogous groups that fold into 7-bladed betapropellers. Structural modeling identified two evolutionary conserved interaction sites in WIPI propellers, one of which may bind phospholipids. Human WIPI-1a has LXXLL signature motifs for nuclear receptor interactions and binds androgen and estrogen receptors in vitro. Strikingly, human WIPI genes were found aberrantly expressed in a variety of matched tumor tissues including kidney, pancreatic and skin cancer. We found that endogenous hWIPI-1 protein colocalizes in part with the autophagosomal marker LC3 at punctate cytoplasmic structures in human melanoma cells. In addition, hWIPI-1 accumulated in large vesicular and cup-shaped structures in the cytoplasm when autophagy was induced by aminoacid deprivation. These cytoplasmic formations were blocked by wortmannin, a classic inhibitor of PI-3 kinase-mediated autophagy. Our data suggest that WIPI proteins share an evolutionary conserved function in autophagy and that autophagic capacity may be compromised in human cancers.

Bone morphogenetic proteins (BMPs) are morphogenetic signaling molecules essential for embryonic patterning. To obtain molecular insight into the influence of BMPs on morphogenesis, we searched for new genes directly activated by BMP signaling. In vitro cultured mouse embryonic stem (ES) cells were used, cultivated in chemically defined growth medium (CDM). CDM-cultured ES cells responded very selectively to stimulation by various mesoderm inducers (BMP2/4, activin A, and basic fibroblast growth factor). BMP2/4 rapidly induced transcript levels of the homeobox genes Msx-1 and Msx-2 and the proto-oncogene JunB, whereas c-jun transcripts displayed delayed albeit prolonged increase. Using differential display cDNA cloning, six direct BMP target genes were identified. These include Id3, which showed strong mRNA induction, and the moderately induced One important stage in the development of the vertebrate embryo is reached when the three germ layers are formed and the body plan gets established during the process of gastrulation (1, 2). Our understanding of the molecular mechanism mediating gastrulation is still insufficient, although over the past years many secreted growth factor-like molecules have been identified to play important roles in these early embryonic events (3). Among these signaling molecules are several members of the BMP 1 family. BMP-related growth factors belong to the TGF-␤-superfamily and have been identified in a wide variety of organisms, ranging from insects to mammals (4). BMPs have originally been isolated for their ability to induce ectopic bone formation when injected under the skin or into the muscle of rodents (5, 6), but meanwhile many of the BMPs have been implicated in a variety of other developmental interactions, including very early embryonic inductive events. For example in Drosophila embryonic dorsal-ventral patterning is partly accomplished through the action of DPP (decapentaplegic) (7,8), which is the BMP member most closely related to mammalian BMP2 and BMP4. At the functional level these factors can substitute for one another in vertebrate and Drosophila embryos. Human BMP4 is able to rescue the dorsalventral pattern defects of dpp null mutants (9), whereas Drosophila DPP protein can induce ectopic bone in mice (10). The striking evolutionary conservation of BMP2, BMP4, and DPP suggested that these molecules play crucial roles in early vertebrate development, as is confirmed meanwhile by many studies. In Xenopus laevis BMP4 ventralizes early mesoderm (11-13) and promotes the differentiation of epidermis from ectoderm (14). Blocking BMP2/4 receptor activity in the ventral part of the embryo eliminates blood formation and dorsalizes the mesoderm (15, 16), whereas in contrast overexpression of BMP4 RNA leads to an increased expression of ventral genes and inhibits the formation of anterior structures (11,12,17). Use of a dominant-negative BMP2/4 receptor to block BMP signals in the ectoderm (18), or disaggregation of ectodermal cells (19), causes differentiation into neural ti...

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