The nuclear actin-related protein of Saccharomyces cerevisiae, Arp4, directly interacts with the histone acetyltransferase Esa1p

Inhibition of the histone methyltransferase EZH2 induces vascular stiffness

Vascular stiffness increases with aging, obesity and hypertension and predicts cardiovascular risk. The levels of histone H3-lysine-27 methylation (H3K27me) and the histone methyltransferase EZH2 both decrease in aging vessels, driving vascular stiffness. The impact of EZH2 inhibitors on vascular stiffness is unknown. We tested the hypothesis that the EZH2 inhibitor GSK126, currently in development for cancer treatment, increases vascular stiffness and explored underlying molecular mechanisms. Young (3 month) and middle-aged (12 month) male mice were treated with GSK126 for 1-2 months and primary human aortic smooth muscle cells (HASMCs) from young male and female donors were treated with GSK126 for 24-48 h. Stiffness was measured in vivo by pulse wave velocity and in vitro by atomic force microscopy (AFM) and vascular structure was quantified histologically. Extracellular matrix proteins were studied by qRT-PCR, immunoblotting, zymography and chromatin immunoprecipitation. GSK126 treatment decreased H3K27 methylation (H3K27me) and increased acetylation (H3K27ac) in mouse vessels and in HASMCs. In GSK126-treated mice, aortic stiffness increased without changes in vascular fibrosis. EZH2 inhibition enhanced elastin fiber degradation and matrix metalloprotease-2 (MMP2) expression. In HASMCs, GSK126 treatment increased synthetic phenotype markers and intrinsic HASMCs stiffness by AFM with altered cytoskeletal structure and increased nuclear actin staining. GSK126 also increased MMP2 protein expression, activity and enrichment of H3K27ac at the MMP2 promoter in HASMCs. GSK126 causes vascular stiffening, inducing MMP2 activity, elastin degradation, and modulation of SMC phenotype and cytoskeletal stiffness. These findings suggest that EZH2 inhibitors used to treat cancer could negatively impact the vasculature by enhancing stiffness and merits examination in human trials.

The Nature of Actin-Family Proteins in Chromatin-Modifying Complexes

Actin is not only one of the most abundant proteins in eukaryotic cells, but also one of the most versatile. In addition to its familiar involvement in enabling contraction and establishing cellular motility and scaffolding in the cytosol, actin has well-documented roles in a variety of processes within the confines of the nucleus, such as transcriptional regulation and DNA repair. Interestingly, monomeric actin as well as actin-related proteins (Arps) are found as stoichiometric subunits of a variety of chromatin remodeling complexes and histone acetyltransferases, raising the question of precisely what roles they serve in these contexts. Actin and Arps are present in unique combinations in chromatin modifiers, helping to establish structural integrity of the complex and enabling a wide range of functions, such as recruiting the complex to nucleosomes to facilitate chromatin remodeling and promoting ATPase activity of the catalytic subunit. Actin and Arps are also thought to help modulate chromatin dynamics and maintain higher-order chromatin structure. Moreover, the presence of actin and Arps in several chromatin modifiers is necessary for promoting genomic integrity and an effective DNA damage response. In this review, we discuss the involvement of actin and Arps in these nuclear complexes that control chromatin remodeling and histone modifications, while also considering avenues for future study to further shed light on their functional importance.

The INO80 ATP-dependent chromatin remodeling complex is a nucleosome spacing factor

The mobilization of nucleosomes by the ATP-dependent remodeler INO80 is quite different from another remodeler (SWI/SNF) that is also involved in gene activation. Unlike that recently shown for SWI/SNF, INO80 is unable to disassemble nucleosomes when remodeling short nucleosomal arrays. Instead, INO80 more closely resembles, although with notable exceptions, the nucleosome spacing activity of ISW2 and ISW1a, which are generally involved in transcription repression. INO80 required a minimum of 33 to 43 bp of extranucleosomal DNA for mobilizing nucleosomes, with 70 bp being optimal. INO80 prefers to move mononucleosomes to the center of DNA, like ISW2 and ISW1a, but does so with higher precision. Unlike ISW2/1a, INO80 does not require the H4 tail for nucleosome mobilization; instead, the H2A histone tail negatively regulates nucleosome movement by INO80. INO80 moved arrays of two or three nucleosomes with 50 or 79 bp of linker DNA closer together, with a final length of ∼30 bp of linker DNA or a repeat length of ∼177 bp. A minimum length of >30 bp of linker DNA was required for nucleosome movement and spacing by INO80 in arrays.

Are histones, tubulin, and actin derived from a common ancestral protein?

Histones and the cytoskeletal components tubulin and actin all act as thermal ratchets, using the energy present in Brownian motion to do work. All three also bind to nucleotides. Here we suggest that histones, tubulin, and actin derive from a common ancestral protein. There is some sequence similarity between histone 2A and the bacterial tubulin homologue FtsZ. Histones and actin also share some sequence similarity in the nucleotides and at phosphate-binding sites. Thus, actin and tubulin may also be related, although this is not obvious from sequence analysis. Indeed, actin and tubulin are closely functionally related and cooperate in many cellular processes. Interestingly, recent advances in nanotechnology suggest that thermal ratchets may be able to impart lifelike properties; thus, the evolution of the ancestral histone, tubulin, and actin thermal ratchet may have been crucial in the development of complexity in living organisms.