Mutagenetic and electron microscopy analysis of actin filament severing by Cordon-Bleu, a WH2 domain protein

The Evolutionary Young Actin Nucleator Cobl Is Important for Proper Amelogenesis

The actin cytoskeleton plays an important role in morphological changes of ameloblasts during the formation of enamel, which is indispensable for teeth to withstand wear, fracture and caries progression. This study reveals that the actin nucleator Cobl is expressed in ameloblasts of mandibular molars during amelogenesis. Cobl expression was particularly pronounced during the secretory phase of the enamel-forming cells. Cobl colocalized with actin filaments at the cell cortex. Importantly, our analyses show an influence of Cobl on both ameloblast morphology and cytoskeletal organization as well as on enamel composition. At P0, Cobl knock-out causes an increased height of ameloblasts and an increased F-actin content at the apical membrane. During the maturation phase, the F-actin density at the apical membrane was instead significantly reduced when compared to WT mice. At the same time, Cobl-deficient mice showed an increased carbon content of the enamel and an increased enamel surface of mandibular molars. These findings demonstrate a decisive influence of the actin nucleator Cobl on the actin cytoskeleton and the morphology of ameloblasts during amelogenesis. Our work thus expands the understanding of the regulation of the actin cytoskeleton during amelogenesis and helps to further elucidate the complex processes of enamel formation during tooth development.

Building the brush border, one microvillus at a time

Microvilli are actin bundle-supported surface protrusions assembled by diverse cell types to mediate biochemical and physical interactions with the external environment. Found on the surface of some of the earliest animal cells, primordial microvilli likely contributed to bacterial entrapment and feeding. Although millions of years of evolution have repurposed these protrusions to fulfill diverse roles such as detection of mechanical or visual stimuli in inner ear hair cells or retinal pigmented epithelial cells, respectively, solute uptake remains a key essential function linked to these structures. In this mini review, we offer a brief overview of the composition and structure of epithelial microvilli, highlight recent discoveries on the growth of these protrusions early in differentiation, and point to fundamental questions surrounding microvilli biogenesis that remain open for future studies.

Phosphorylation of the WH2 domain in yeast Las17/WASP regulates G-actin binding and protein function during endocytosis

Actin nucleation is the key rate limiting step in the process of actin polymerization, and tight regulation of this process is critical to ensure actin filaments form only at specific times and at defined regions of the cell. WH2 domains are short sequence motifs found in many different actin binding proteins including WASP family proteins which regulate the actin nucleating complex Arp2/3. In this study we reveal a phosphorylation site, Serine 554, within the WH2 domain of the yeast WASP homologue Las17. Both phosphorylation and a phospho-mimetic mutation reduce actin monomer binding affinity while an alanine mutation, generated to mimic the non-phosphorylated state, increases actin binding affinity. The effect of these mutations on the Las17-dependent process of endocytosis in vivo was analysed and leads us to propose that switching of Las17 phosphorylation states may allow progression through distinct phases of endocytosis from site assembly through to the final scission stage. While the study is focused on Las17, the sole WASP family protein in yeast, our results have broad implications for our understanding of how a key residue in this conserved motif can underpin the many different actin regulatory roles with which WH2 domains have been associated.

Proteomic resolution of IGFN1 complexes reveals a functional interaction with the actin nucleating protein COBL

The Igfn1 gene produces multiple proteins by alternative splicing predominantly expressed in skeletal muscle. Igfn1 deficient clones derived from C2C12 myoblasts show reduced fusion index and morphological differences compared to control myotubes. Here, we first show that G:F actin ratios are significantly higher in differentiating IGFN1-deficient C2C12 myoblasts, suggesting that fusion and differentiation defects are underpinned by deficient actin remodelling. We obtained pull-downs from skeletal muscle with IGFN1 fragments and applied a proteomics approach. The proteomic composition of IGFN1 complexes identified the cytoskeleton and an association with the proteasome as the main networks. The actin nucleating protein COBL was selected for further validation. COBL is expressed in C2C12 myoblasts from the first stages of myoblast fusion but not in proliferating cells. COBL is also expressed in adult muscle and, as IGFN1, localizes to the Z-disc. We show that IGFN1 interacts, stabilizes and colocalizes with COBL and prevents the ability of COBL to form actin ruffles in COS7 cells. COBL loss of function C2C12-derived clones are able to fuse, therefore indicating that COBL or the IGFN1/COBL interaction are not essential for myoblast fusion.

Cellular consequences of arginine methylation

Arginine methylation is a ubiquitous post-translational modification. Three predominant types of arginine-guanidino methylation occur in Eukarya: mono (Rme1/MMA), symmetric (Rme2s/SDMA), and asymmetric (Rme2a/ADMA). Arginine methylation frequently occurs at sites of protein-protein and protein-nucleic acid interactions, providing specificity for binding partners and stabilization of important biological interactions in diverse cellular processes. Each methylarginine isoform-catalyzed by members of the protein arginine methyltransferase family, Type I (PRMT1-4,6,8) and Type II (PRMT5,9)-has unique downstream consequences. Methylarginines are found in ordered domains, domains of low complexity, and in intrinsically disordered regions of proteins-the latter two of which are intimately connected with biological liquid-liquid phase separation. This review highlights discoveries illuminating how arginine methylation affects genome integrity, gene transcription, mRNA splicing and mRNP biology, protein translation and stability, and phase separation. As more proteins and processes are found to be regulated by arginine methylation, its importance for understanding cellular physiology will continue to grow.

Regulation of MRTF-A by JMY via a nucleation-independent mechanism

Background

MRTF-A (myocardin-related transcription factor A) is a coactivator for SRF-mediated gene expression. The activity of MRTF-A is critically dependent on the dissociation of G-actin from N-terminal RPEL motifs. MRTF-SRF induction often correlates with enhanced polymerization of F-actin. Here we investigate MRTF regulation by the multifunctional JMY protein, which contains three WASP/verprolin homology 2 (WH2/V) domains and facilitates Arp2/3-dependent and -independent actin nucleation.

Methods

Co-immunoprecipitation experiments, immunofluorescence and luciferase reporter assays were combined with selective inhibitors to investigate the effect of JMY and its domains on MRTF-A in NIH 3 T3 mouse fibroblasts.

Results

JMY induced MRTF-A transcriptional activity and enhanced its nuclear translocation. Unexpectedly, MRTF-A was hyperactivated when the Arp2/3-recruiting CA region of JMY was deleted or mutated, suggesting an autoinhibitory mechanism for full-length JMY. Moreover, isolated WH2/V domains which are unable to nucleate actin were sufficient for nuclear accumulation and SRF activation. Recombinant WH2/V regions of JMY biochemically competed with MRTF-A for actin binding. Activation of MRTF-A by JMY was unaffected by Arp3 knockdown, by an Arp2/3 inhibitor, and by latrunculin which disassembles cellular F-actin. Restriction of JMY to the nucleus abrogated its MRTF-A activation. Finally, JMY RNAi reduced basal and stimulated transcriptional activation via MRTF-A.

Conclusions

Our results suggest that JMY activates MRTF-SRF independently of F-actin via WH2/V-mediated competition with the RPEL region for G-actin binding in the cytoplasm. Furthermore, the C-terminal region facilitates an autoinhibitory effect on full-length JMY, possibly by intramolecular folding.

Regulation of actin-based apical structures on epithelial cells

Cells of transporting epithelia are characterized by the presence of abundant F-actin-based microvilli on their apical surfaces. Likewise, auditory hair cells have highly reproducible rows of apical stereocilia (giant microvilli) that convert mechanical sound into an electrical signal. Analysis of mutations in deaf patients has highlighted the critical components of tip links between stereocilia, and related structures that contribute to the organization of microvilli on epithelial cells have been found. Ezrin/radixin/moesin (ERM) proteins, which are activated by phosphorylation, provide a critical link between the plasma membrane and underlying actin cytoskeleton in surface structures. Here, we outline recent insights into how microvilli and stereocilia are built, and the roles of tip links. Furthermore, we highlight how ezrin is locally regulated by phosphorylation, and that this is necessary to maintain polarity. Localized phosphorylation is achieved through an intricate coincidence detection mechanism that requires the membrane lipid phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂] and the apically localized ezrin kinase, lymphocyte-oriented kinase (LOK, also known as STK10) or Ste20-like kinase (SLK). We also discuss how ezrin-binding scaffolding proteins regulate microvilli and how, despite these significant advances, it remains to be discovered how the cell polarity program ultimately interfaces with these processes.

Latrunculin A Accelerates Actin Filament Depolymerization in Addition to Sequestering Actin Monomers

Latrunculin A (LatA), a toxin from the red sea sponge Latrunculia magnifica, is the most widely used reagent to depolymerize actin filaments in experiments on live cells. LatA binds actin monomers and sequesters them from polymerization [1, 2]. Low concentrations of LatA result in rapid (tens of seconds) disassembly of actin filaments in animal [3] and yeast cells [2]. Depolymerization is usually assumed to result from sequestration of actin monomers. Our observations of single-muscle actin filaments by TIRF microscopy showed that LatA bound ATP-actin monomers with a higher affinity (K_d = 0.1 μM) than ADP-P_i-actin (K_d = 0.4 μM) or ADP-actin (K_d = 4.7 μM). LatA also slowly severed filaments and increased the depolymerization rate at both ends of filaments freshly assembled from ATP-actin to the rates of ADP-actin. This rate plateaued at LatA concentrations >60 μM. LatA did not change the depolymerization rates of ADP- actin filaments or ADP-P_i-actin filaments generated with 160 mM phosphate in the buffer. LatA did not increase the rate of phosphate release from bulk samples of filaments assembled from ATP-actin. Thermodynamic analysis showed that LatA binds weakly to actin filaments with a K_d >100 μM. We propose that concentrations of LatA much lower than this K_d promote phosphate dissociation only from both ends of filaments, resulting in depolymerization limited by the rate of ADP-actin dissociation. Thus, one must consider both rapid actin depolymerization and severing in addition to sequestering actin monomers when interpreting the effects of LatA on cells.

Enhanced Depolymerization of Actin Filaments by ADF/Cofilin and Monomer Funneling by Capping Protein Cooperate to Accelerate Barbed-End Growth

A living cell’s ability to assemble actin filaments in intracellular motile processes is directly dependent on the availability of polymerizable actin monomers, which feed polarized filament growth [1, 2]. Continued generation of the monomer pool by filament disassembly is therefore crucial. Disassemblers like actin depolymerizing factor (ADF)/cofilin and filament cappers like capping protein (CP) are essential agonists of motility [3, 4, 5, 6, 7, 8], but the exact molecular mechanisms by which they accelerate actin polymerization at the leading edge and filament turnover has been debated for over two decades [9, 10, 11, 12]. Whereas filament fragmentation by ADF/cofilin has long been demonstrated by total internal reflection fluorescence (TIRF) [13, 14], filament depolymerization was only inferred from bulk solution assays [15]. Using microfluidics-assisted TIRF microscopy, we provide the first direct visual evidence of ADF’s simultaneous severing and rapid depolymerization of individual filaments. Using a conceptually novel assay to directly visualize ADF’s effect on a population of pre-assembled filaments, we demonstrate how ADF’s enhanced pointed-end depolymerization causes an increase in polymerizable actin monomers, thus promoting faster barbed-end growth. We further reveal that ADF-enhanced depolymerization synergizes with CP’s long-predicted “monomer funneling” [16] and leads to skyrocketing of filament growth rates, close to estimated lamellipodial rates. The “funneling model” hypothesized, on thermodynamic grounds, that at high enough extent of capping, the few non-capped filaments transiently grow much faster [15], an effect proposed to be very important for motility. We provide the first direct microscopic evidence of monomer funneling at the scale of individual filaments. These results significantly enhance our understanding of the turnover of cellular actin networks.

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ADF enhances barbed- and pointed-end depolymerization of actin filaments

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Capping protein funnels monomers from all pointed ends to the few non-capped barbed ends

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ADF and capping protein synergy leads to skyrocketing of filament elongation rates

Impact of cordon-bleu expression on actin cytoskeleton architecture and dynamics

Cordon-bleu (COBL) is a multifunctional WASP-Homology 2 (WH2) domain-containing protein implicated in a wide variety of cellular functions ranging from dendritic arborization in neurons to the assembly of microvilli on the surface of transporting epithelial cells. In vitro biochemical studies suggest that COBL is capable of nucleating and severing actin filaments, among other activities. How the multiple activities of COBL observed in vitro contribute to its function in cells remains unclear. Here, we used live imaging to evaluate the impact of COBL expression on the actin cytoskeleton in cultured cells. We found that COBL induces the formation of dynamic linear actin structures throughout the cytosol. We also found that stabilizing these dynamic structures with the parallel actin-bundling protein espin slows down their turnover and enables the robust formation of self-supported protrusions on the dorsal cell surface. Super-resolution imaging revealed a global remodeling of the actin cytoskeleton in cells expressing these two factors. Taken together, these results provide insight as to how COBL contributes to the assembly of actin-based structures such as epithelial microvilli. © 2016 Wiley Periodicals, Inc.

Quarterly intrinsic disorder digest (January-February-March, 2014)

This is the 5^th issue of the Digested Disorder series that represents a reader's digest of the scientific literature on intrinsically disordered proteins. We continue to use only 2 criteria for inclusion of a paper to this digest: The publication date (a paper should be published within the covered time frame) and the topic (a paper should be dedicated to any aspect of protein intrinsic disorder). The current digest issue covers papers published during the first quarter of 2014; i.e., during the period of January, February, and March of 2014. Similar to previous issues, the papers are grouped hierarchically by topics they cover, and for each of the included papers a short description is given on its major findings.

Cordon Bleu serves as a platform at the basal region of microvilli, where it regulates microvillar length through its WH2 domains

The actin nucleator Cordon Bleu (Cobl) is localized to the basal region of microvilli of epithelial cells, where it regulates microvilli length through its WH2 domains. The COBL domain recruits several BAR-containing proteins, including PACSIN 2 and ASAP1, suggesting a role in coordinating microvillar structure with membrane traffic.

Cordon Bleu (Cobl) is a WH2-containing protein believed to act as an actin nucleator. We show that it has a very specific localization in epithelial cells at the basal region of microvilli, a localization unlikely to be involved in actin nucleation. The protein is localized by a central region between the N-terminal COBL domain and the three C-terminal WH2 domains. Ectopic expression of Cobl shortens apical microvilli, and this requires functional WH2 domains. Proteomic studies reveal that the COBL domain binds several BAR-containing proteins, including SNX9, PACSIN 2/syndapin 2, and ASAP1. ASAP1 is recruited to the base of microvilli by binding the COBL domain through its SH3. We propose that Cobl is localized to the basal region of microvilli both to participate in length regulation and to recruit BAR proteins that associate with the curved membrane found at the microvillar base.