Heritable transcriptional defects from aberrations of nuclear architecture.

Transcriptional heterogeneity due to plasticity of the epigenetic state of chromatin contributes to tumour evolution, metastasis and drug resistance. However, the mechanisms that cause this epigenetic variation are incompletely understood. Here we identify micronuclei and chromosome bridges, aberrations in the nucleus common in cancer, as sources of heritable transcriptional suppression.

Breakage of cytoplasmic chromosomes by pathological DNA base excision repair.

Chromothripsis is a catastrophic mutational process that promotes tumorigenesis and causes congenital disease. Chromothripsis originates from aberrations of nuclei called micronuclei or chromosome bridges. These structures are associated with fragile nuclear envelopes that spontaneously rupture, leading to DNA damage when chromatin is exposed to the interphase cytoplasm.

Factors promoting nuclear envelope assembly independent of the canonical ESCRT pathway.

The nuclear envelope (NE) undergoes dynamic remodeling to maintain NE integrity, a process involving the inner nuclear membrane protein LEM2 recruiting CHMP7/Cmp7 and then ESCRT-III. However, prior work has hinted at CHMP7/ESCRT-independent mechanisms. To identify such mechanisms, we studied NE assembly in Schizosaccharomyces japonicus, a fission yeast that undergoes partial mitotic NE breakdown and reassembly.

A Tubulin Binding Switch Underlies Kip3/Kinesin-8 Depolymerase Activity.

Kinesin-8 motors regulate the size of microtubule structures, using length-dependent accumulation at the plus end to preferentially disassemble long microtubules. Despite extensive study, the kinesin-8 depolymerase mechanism remains under debate. Here, we provide evidence for an alternative, tubulin curvature-sensing model of microtubule depolymerization by the budding yeast kinesin-8, Kip3.

Regulation of the formin Bnr1 by septins anda MARK/Par1-family septin-associated kinase.

Formin-family proteins promote the assembly of linear actin filaments and are required to generate cellular actin structures, such as actin stress fibers and the cytokinetic actomyosin contractile ring. Many formin proteins are regulated by an autoinhibition mechanism involving intramolecular binding of a Diaphanous inhibitory domain and a Diaphanous autoregulatory domain. However, the activation mechanism for these Diaphanous-related formins (DRFs) is not completely understood.

Mutant profilin suppresses mutant actin-dependent mitochondrial phenotype in Saccharomyces cerevisiae.

In the Saccharomyces cerevisiae actin-profilin interface, Ala(167) of the actin barbed end W-loop and His(372) near the C terminus form a clamp around a profilin segment containing residue Arg(81) and Tyr(79). Modeling suggests that altering steric packing in this interface regulates actin activity. An actin A167E mutation could increase interface crowding and alter actin regulation, and A167E does cause growth defects and mitochondrial dysfunction.

A potential yeast actin allosteric conduit dependent on hydrophobic core residues val-76 and trp-79.

Intramolecular allosteric interactions responsible for actin conformational regulation are largely unknown. Previous work demonstrated that replacing yeast actin Val-76 with muscle actin Ile caused decreased nucleotide exchange. Residue 76 abuts Trp-79 in a six-residue linear array beginning with Lys-118 on the surface and ending with His-73 in the nucleotide cleft.

Actin isoform-specific conformational differences observed with hydrogen/deuterium exchange and mass spectrometry.

Actin can exist in multiple conformations necessary for normal function. Actin isoforms, although highly conserved in sequence, exhibit different biochemical properties and cellular roles. We used amide proton hydrogen/deuterium (HD) exchange detected by mass spectrometry to analyze conformational differences between Saccharomyces cerevisiae and muscle actins in the G and F forms to gain insight into these differences.

Role of intermonomer ionic bridges in the stabilization of the actin filament.

Filament formation is required for most of the functions of actin. However, the intermonomer interactions that stabilize F-actin have not been elucidated because of a lack of an F-actin crystal structure. The Holmes muscle actin model suggests that an ionic interaction between Arg-39 of one monomer and Glu-167 of an adjacent monomer in the same strand contributes to this stabilization.