{beta}CaMKII regulates actin assembly and structure.
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ABSTRACT: Ca(2+)-Calmodulin-dependent protein kinase II (CaMKII) is an abundant synaptic protein that was recently shown to regulate the organization of actin filaments leading to structural modifications of synapses. CaMKII is a dodecameric complex with a special architecture that provides it with unique potential for organizing the actin cytoskeleton. We report using biochemical assays that the beta isoform of CaMKII binds to and bundles actin filaments, and the disposition of betaCaMKII within the actin bundles was revealed by cryoelectron tomography. In addition, betaCaMKII was found to inhibit actin polymerization, suggesting that it either serves as a capping protein or binds monomeric actin, reducing the amount of freely available monomers to nucleate polymer assembly. By means of fluorescent cross-correlation spectroscopy, we determined that betaCaMKII does indeed bind to monomeric actin, reaching saturation at a stoichiometry of 12:1 actin monomers per betaCaMKII holoenzyme with a binding constant of 2.4 x 10(5) m(-1). In cells, betaCaMKII has a dual functional role; it can sequester monomeric actin to reduce actin polymerization and can also bundle actin filaments. Together, these effects would impact both the dynamics of actin filament assembly and enhance the rigidity of the filaments once formed, significantly impacting the structure of synapses.
Project description:Polymerized b-actin may provide a structural basis for chromatin accessibility. Nuclear actin transport into the nucleus can determine mesenchymal stem cell (MSC) differentiative outcomes through regulated control of gene expression. Using MSC, we show that inhibiting Arp2/3 directed secondary actin branching with CK666, which results in decreased nuclear actin structure, significantly alters chromatin access measured with ATAC-seq at 24 h. The ATAC-seq results due to CK666 are distinct from those caused by cytochalasin D (CytoD), which increases nuclear actin structure. Nuclear visualization shows Arp2/3 inhibition is associated with decreased pericentric H3K9me3 marks. CytoD, in contrast, induces relocation of H3K27me3 marks away from the inner membrane. Treatment induced alterations in the chromatin landscape at 24 hours prompts differential gene expression associated with decreased proliferation and increased differentiation. Further, knockdown of the non-enzymatic monomeric actin binding protein, Arp4, leads to extensive chromatin unpacking, but only a modest change in transcription, indicating an active role for actin-Arp4 in transcription. These data indicate that dynamic actin remodeling can regulate chromatin interactions.
Project description:Polymerized B-actin may provide a structural basis for chromatin accessibility and actin transport into the nucleus can guide mesenchymal stem cell (MSC) differentiation. Using MSC, we show that using CK666 to inhibit Arp2/3 directed secondary actin branching results in decreased nuclear actin structure, and significantly alters chromatin access measured with ATACseq at 24 h. The ATAC-seq results due to CK666 are distinct from those caused by cytochalasin D (CytoD), which enhances nuclear actin structure. In addition, nuclear visualization shows Arp2/3 inhibition decreases pericentric H3K9me3 marks. CytoD, alternatively, induces redistribution of H3K27me3 marks centrally. Such alterations in chromatin landscape are consistent with differential gene expression associated with distinctive differentiation patterns. Further, knockdown of the non-enzymatic monomeric actin binding protein, Arp4, leads to extensive chromatin unpacking, but only a modest increase in transcription, indicating an active role for actin-Arp4 in transcription. These data indicate that dynamic actin remodeling can regulate chromatin interactions.