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The idle assembly checkpoint acts to delay chromosome segregation until all duplicated sister chromatids are captured by the mitotic spindle. This pathway ensures that each daughter cell receives a complete copy of the genome. The high fidelity and robustness of this process have made it a subject of intense study in both the experimental and computational realms. A significant number of checkpoint proteins have been identified but how they orchestrate the communication between local spindle attachment and global cytoplasmic signalling to delay segregation is not yet understood. Here, we propose a systems view of the spindle assembly checkpoint to focus attention on the key regulators of the dynamics of this pathway. These regulators in turn have been the subject of detailed cellular measurements and computational modelling to connect molecular function to the dynamics of spindle assembly checkpoint signalling. A review of these efforts reveals the insights provided by such approaches and underscores the need for further interdisciplinary studies to reveal in full the quantitative underpinnings of this cellular control pathway. [ABSTRACT FROM AUTHOR]

Centromere protein F (CENP-F) (or mitosin) accumulates to become an abundant nuclear protein in G2, assembles at kinetochores in late G2, remains kinetochore-bound until anaphase, and is degraded at the end of mitosis. Here we show that the absence of nuclear CENP-F does not affect cell cycle progression in S and G2. In a subset of CENP-F depleted cells, kinetochore assembly fails completely, thereby provoking massive chromosome mis-segregation. In contrast, the majority of CENP-F depleted cells exhibit a strong mitotic delay with reduced tension between kinetochores of aligned, bi-oriented sister chromatids and decreased stability of kinetochore microtubules. These latter kinetochores generate mitotic checkpoint signaling when unattached, recruiting maximum levels of Mad2. Use of YFP-marked Mad1 reveals that throughout the mitotic delay some aligned, CENP-F depleted kinetochores continuously recruit Mad1. Others rebind YFP-Mad1 intermittently so as to produce ‘twinkling’, demonstrating cycles of mitotic checkpoint reactivation and silencing and a crucial role for CENP-F in efficient assembly of a stable microtubule–kinetochore interface. [ABSTRACT FROM AUTHOR]

Discusses the function of the protein called plectin at the cellular level. Phenotype of plectin-deficient cells; How plectin binds actin and phosphatidylinositol 4,5-biphosphate (PIP...); Analysis of plectin's modulation of the actin cytoskeleton.

Focuses on the regulation of filament bundles and cytoskeletal networks by polycations and proteins. Names of the three types of protein filaments; Details on the viscoelastic properties of networks formed by F-actin; Features of biological tissues.

While the physical nature of the epigenetic mark defining the location of the centromere on the chromosome is not known, nucleosomes in which CENP-A replaces histone H3 are at the foundation of the centromeric chromatin. Hydrogen/deuterium exchange mass spectrometry is now used to show that assembly into nucleosomes imposes stringent conformational constraints, reducing solvent accessibility in almost all histone regions by >L3 orders of magnitude. Nucleosomes assembled with CENP-A are substantially more conformationally rigid than those assembled with histone H3, independent of DNA template. Substitution of the CATD, comprised of the loop 1 and α2 helix of the CENP-A histone fold domain, into histone H3 generates the same more rigid nucleosome as does CENP-A. In human cells, while depletion of CENPA is lethal, recruitment of normal levels of kinetochore proteins, centromere-generated mitotic checkpoint signaling, normal chromosome segregation and viability can be rescued by this chimeric H3 containing the CATD. The CATD, itself, is confined to the structured "core" of the nucleosome demonstrating that much (if not all) of the essential features of CENP-A are within the rigid core of the nucleosome. These data support a model of centromere identity maintained by a unique nucleosome structure that serves to distinguish the centromere from the rest of the chromosome. [ABSTRACT FROM AUTHOR]

Summary: Active centromeres are marked by nucleosomes assembled with CENP-A, a centromere-specific histone H3 variant. The CENP-A centromere targeting domain (CATD), comprised of loop 1 and the α2 helix within the histone fold, is sufficient to target histone H3 to centromeres and to generate the same conformational rigidity to the initial subnucleosomal heterotetramer with histone H4 as does CENP-A. We now show in human cells and in yeast that depletion of CENP-A is lethal, but recruitment of normal levels of kinetochore proteins, centromere-generated mitotic checkpoint signaling, chromosome segregation, and viability can be rescued by histone H3 carrying the CATD. These data offer direct support for centromere identity maintained by a unique nucleosome that serves to distinguish the centromere from the rest of the chromosome. [Copyright &y& Elsevier]