Publication highlights

The MRTF-SRF and the YAP-TEAD transcriptional regulatory networks both respond to extracellular signals and mechanical stimuli: the MRTFs are controlled directly by G-actin, while YAP activity is somehow potentiated by F-actin. Cancer-associated fibroblasts play an important pro-invasive role in stimulating cancer progression, and previous studies have shown that this involves YAP-TEAD signalling. This paper shows that CAFs also exhibit mechanically-dependent MRTF activation, which is also required for their contractile and pro-invasive activity. The two pathways are mutually dependent, requiring recruitment of MRTF and YAP to DNA via their respective DNA-binding partners, and reflecting their ability to control cytoskeletal gene expression.

Unlike kinases, PPP-family phosphatases such as PP1 have little intrinsic specificity. PP1 acts in partnership with over 200 different PP1-interacting proteins, but it has remained unclear how they might confer sequence-specificity on PP1. We used proteomics to identify dozens of candidate Phactr1/PP1 substrates, and used structural and biochemical approaches to show that the Phactr1/PP1 holoenzyme is sequence-specific. Phactr1 binding reshapes the PP1 hydrophobic groove, thereby creating a novel composite hydrophobic surface for substrate recognition. This study explains how cofactors can enhance the reactivity of PP1 toward specific substrates, and suggests a way forward for the development of PP1 holoenzyme-specific inhibitors.

This paper shows that the ArhGAP9/12/15/27 and ArhGAP32/33 families of rhoGAPs are RPEL proteins whose activity is coupled to G-actin concentration. G-actin forms a 1:1 complex with these ArhGAPs, interacting with an RPEL motif located between the PH and GAP domains, thereby inhibiting their GAP activity. Mutations that block G-actin binding exhibit elevated GAP activity towards their substrate GTPases Rac and Cdc42. Strikingly, treatment of cells with drugs enhancing or inhibiting G-actin/ArhGAP interaction has corresponding effects on Rac GTP loading. These results establish a novel homeostatic feedback loop, in which ArhGAP12-family (and presumably ArhGAP32-family) GAP activity increases when G-actin levels become limiting.