Retraction Note: Formin' a perinuclear actin cage in confined migration.

Given that the authors of 'FMN2 makes perinuclear actin to protect nuclei during confined migration and promote metastasis' (Skau et al. Cell 167, 1571-1585; 2016) have retracted their paper, I wish to retract this Research Highlight, which discussed the findings reported in that study.

A poised chromatin platform for TGF-ß access to master regulators.

Specific chromatin marks keep master regulators of differentiation silent yet poised for activation by extracellular signals. We report that nodal TGF-ß signals use the poised histone mark H3K9me3 to trigger differentiation of mammalian embryonic stem cells. Nodal receptors induce the formation of companion Smad4-Smad2/3 and TRIM33-Smad2/3 complexes. The PHD-Bromo cassette of TRIM33 facilitates binding of TRIM33-Smad2/3 to H3K9me3 and H3K18ac on the promoters of mesendoderm regulators Gsc and Mixl1. The crystal structure of this cassette, bound to histone H3 peptides, illustrates that PHD recognizes K9me3, and Bromo binds an adjacent K18ac. The interaction between TRIM33-Smad2/3 and H3K9me3 displaces the chromatin-compacting factor HP1γ, making nodal response elements accessible to Smad4-Smad2/3 for Pol II recruitment. In turn, Smad4 increases K18 acetylation to augment TRIM33-Smad2/3 binding. Thus, nodal effectors use the H3K9me3 mark as a platform to switch master regulators of stem cell differentiation from the poised to the active state.

A Smad action turnover switch operated by WW domain readers of a phosphoserine code.

When directed to the nucleus by TGF-ß or BMP signals, Smad proteins undergo cyclin-dependent kinase 8/9 (CDK8/9) and glycogen synthase kinase-3 (GSK3) phosphorylations that mediate the binding of YAP and Pin1 for transcriptional action, and of ubiquitin ligases Smurf1 and Nedd4L for Smad destruction. Here we demonstrate that there is an order of events-Smad activation first and destruction later-and that it is controlled by a switch in the recognition of Smad phosphoserines by WW domains in their binding partners. In the BMP pathway, Smad1 phosphorylation by CDK8/9 creates binding sites for the WW domains of YAP, and subsequent phosphorylation by GSK3 switches off YAP binding and adds binding sites for Smurf1 WW domains. Similarly, in the TGF-ß pathway, Smad3 phosphorylation by CDK8/9 creates binding sites for Pin1 and GSK3, then adds sites to enhance Nedd4L binding. Thus, a Smad phosphoserine code and a set of WW domain code readers provide an efficient solution to the problem of coupling TGF-ß signal delivery to turnover of the Smad signal transducers.

Nuclear CDKs drive Smad transcriptional activation and turnover in BMP and TGF-beta pathways.

TGF-beta and BMP receptor kinases activate Smad transcription factors by C-terminal phosphorylation. We have identified a subsequent agonist-induced phosphorylation that plays a central dual role in Smad transcriptional activation and turnover. As receptor-activated Smads form transcriptional complexes, they are phosphorylated at an interdomain linker region by CDK8 and CDK9, which are components of transcriptional mediator and elongation complexes. These phosphorylations promote Smad transcriptional action, which in the case of Smad1 is mediated by the recruitment of YAP to the phosphorylated linker sites. An effector of the highly conserved Hippo organ size control pathway, YAP supports Smad1-dependent transcription and is required for BMP suppression of neural differentiation of mouse embryonic stem cells. The phosphorylated linker is ultimately recognized by specific ubiquitin ligases, leading to proteasome-mediated turnover of activated Smad proteins. Thus, nuclear CDK8/9 drive a cycle of Smad utilization and disposal that is an integral part of canonical BMP and TGF-beta pathways.

MAL and ternary complex factor use different mechanisms to contact a common surface on the serum response factor DNA-binding domain.

The transcription factor serum response factor (SRF) interacts with its cofactor, MAL/MKL1, a member of the myocardin-related transcription factor (MRTF) family, through its DNA-binding domain. We define a seven-residue sequence within the conserved MAL B1 region essential and sufficient for complex formation. The neighboring Q-box sequence facilitates this interaction. The B1 and Q-box regions also have antagonistic effects on MAL nuclear import, but the residues involved are largely distinct. Both MAL and the ternary complex factor (TCF) family of SRF cofactors interact with a hydrophobic groove and pocket on the SRF DNA-binding domain. Unlike the TCFs, however, interaction of MAL with SRF is impaired by SRF alphaI-helix mutations that reduce DNA bending in the SRF-DNA complex. A clustered SRF alphaI-helix mutation strongly impairs MAL-SRF complex formation but does not affect DNA distortion in the MAL-SRF complex. MAL-SRF complex formation is facilitated by DNA binding. DNase I footprinting indicates that in the SRF-MAL complex MAL directly contacts DNA. These contacts, which flank the DNA sequences protected from DNase I by SRF, are required for effective MAL-SRF complex formation in gel mobility shift assays. We propose a model of MAL-SRF complex formation in which MAL interacts with SRF by the addition of a beta-strand to the SRF DNA-binding domain beta-sheet region, while SRF-induced DNA bending facilitates MAL-DNA contact.

Actin dynamics control SRF activity by regulation of its coactivator MAL.

Rho GTPases regulate the transcription factor SRF via their ability to induce actin polymerization. SRF activity responds to G actin, but the mechanism of this has remained unclear. We show that Rho-actin signaling regulates the subcellular localization of the myocardin-related SRF coactivator MAL, rearranged in t(1;22)(p13;q13) AML. The MAL-SRF interaction displays the predicted properties of a Rho-regulated SRF cofactor. MAL is predominantly cytoplasmic in serum-starved cells, but accumulates in the nucleus following serum stimulation. Activation of the Rho-actin signaling pathway is necessary and sufficient to promote MAL nuclear accumulation. MAL N-terminal sequences, including two RPEL motifs, are required for the response to signaling, while other regions mediate its nuclear export (or cytoplasmic retention) and nuclear import. MAL associates with unpolymerized actin through its RPEL motifs. Constitutively cytoplasmic MAL derivatives interfere with MAL redistribution and Rho-actin signaling to SRF. MAL associates with several SRF target promoters regulated via the Rho-actin pathway.