The synaptonemal complex (SC) is a polymer that spans ~100 nm between paired homologous chromosomes during meiosis. Its striated, periodic appearance in electron micrographs led to the idea that transverse filaments within this structure ‘crosslink’ the axes of homologous chromosomes, stabilizing their pairing. SC proteins can also form polycomplexes, three-dimensional lattices that recapitulate the periodic structure of SCs but do not associate with chromosomes. Here we provide evidence that SCs and polycomplexes contain mobile subunits and that their assembly is promoted by weak hydrophobic interactions, indicative of a liquid crystalline phase. We further show that in the absence of recombination intermediates, polycomplexes recapitulate the dynamic localization of pro-crossover factors during meiotic progression, revealing how the SC might act as a conduit to regulate chromosome-wide crossover distribution. Properties unique to liquid crystals likely enable long-range signal transduction along meiotic chromosomes and underlie the rapid evolution of SC proteins.DOI: http://dx.doi.org/10.7554/eLife.21455.001

Structure formation and constant reorganization of the actin cytoskeleton are key requirements for the function of living cells.Here we show that a minimal reconstituted system consisting of actin filaments, crosslinking molecules and molecular-motor filaments exhibits a generic mechanism of structure formation, characterized by a broad distribution of cluster sizes. We demonstrate that the growth of the structures depends on the intricate balance between crosslinker-induced stabilization and simultaneous destabilization by molecular motors, a mechanism analogous to nucleation and growth in passive systems. We also show that the intricate interplay between force generation, coarsening and connectivity is responsible for the highly dynamic process of structure formation in this heterogeneous active gel, and that these competing mechanisms result in anomalous transport, reminiscent of 1 intracellular dynamics.

During meiosis, each pair of homologous chromosomes typically undergoes at least one crossover (crossover assurance), but these exchanges are strictly limited in number and widely spaced along chromosomes (crossover interference). The molecular basis for this chromosome-wide regulation remains mysterious. A family of meiotic RING finger proteins has been implicated in crossover regulation across eukaryotes. Caenorhabditis elegans expresses four such proteins, of which one (ZHP-3) is known to be required for crossovers. Here we investigate the functions of ZHP-1, ZHP-2, and ZHP-4. We find that all four ZHP proteins, like their homologs in other species, localize to the synaptonemal complex, an unusual, liquid crystalline compartment that assembles between paired homologs. Together they promote accumulation of pro-crossover factors, including ZHP-3 and ZHP-4, at a single recombination intermediate, thereby patterning exchanges along paired chromosomes. These proteins also act at the top of a hierarchical, symmetry-breaking process that enables crossovers to direct accurate chromosome segregation.

When cells enter meiosis, their chromosomes reorganize as linear arrays of chromatin loops anchored to a central axis. Meiotic chromosome axes form a platform for the assembly of the synaptonemal complex (SC) and play central roles in other meiotic processes, including homologous pairing, recombination, and chromosome segregation. However, little is known about the 3D organization of components within the axes, which include cohesin complexes and additional meiosis-specific proteins. Here, we investigate the molecular organization of meiotic chromosome axes in Caenorhabditis elegans through STORM (stochastic optical reconstruction microscopy) and PALM (photo-activated localization microscopy) superresolution imaging of intact germ-line tissue. By tagging one axis protein (HIM-3) with a photoconvertible fluorescent protein, we established a spatial reference for other components, which were localized using antibodies against epitope tags inserted by CRISPR/Cas9 genome editing. Using 3D averaging, we determined the position of all known components within synapsed chromosome axes to high spatial precision in three dimensions. We find that meiosis-specific HORMA domain proteins span a gap between cohesin complexes and the central region of the SC, consistent with their essential roles in SC assembly. Our data further suggest that the two different meiotic cohesin complexes are distinctly arranged within the axes: Although cohesin complexes containing the kleisin REC-8 protrude above and below the plane defined by the SC, complexes containing COH-3 or -4 kleisins form a central core, which may physically separate sister chromatids. This organization may help to explain the role of the chromosome axes in promoting interhomolog repair of meiotic double-strand breaks by inhibiting intersister repair.

The synaptonemal complex (SC) is a tripartite protein scaffold that forms between homologous chromosomes during meiosis. Although the SC is essential for stable homologue pairing and crossover recombination in diverse eukaryotes, it is unknown how individual components assemble into the highly conserved SC structure. Here we report the biochemical identification of two new SC components, SYP-5 and SYP-6, in Caenorhabditis elegans. SYP-5 and SYP-6 are paralogous to each other and play redundant roles in synapsis, providing an explanation for why these genes have evaded previous genetic screens. Superresolution microscopy reveals that they localize between the chromosome axes and span the width of the SC in a head-to-head manner, similar to the orientation of other known transverse filament proteins. Using genetic redundancy and structure–function analyses to truncate C-terminal tails of SYP-5/6, we provide evidence supporting the role of SC in both limiting and promoting crossover formation.

Simplified in vitro systems are ideally suited for studying the principle mechanisms of the contraction of cytoskeletal actin systems. To shed light on the dependence of the contraction mechanism on the nature of the crosslinking proteins, we study reconstituted in vitro active actin networks on different length scales ranging from the molecular organization to the macroscopic contraction. Distinct contraction mechanisms are observed in polar and apolar crosslinked active gels whereas composite active gels crosslinked in a polar and apolar fashion at the same time exhibit both mechanisms simultaneously. In polar active actin/fascin networks initially bundles are formed which are then rearranged. In contrast, apolar cortexillin-I crosslinked active gels are bundled only after reorganization of actin filaments by myosin-II motor filaments.

Self organization mechanisms are essential for the cytoskeleton to adapt to the requirements of living cells. They rely on the intricate interplay of cytoskeletal filaments, crosslinking proteins and molecular motors. Here we present an in vitro minimal model system consisting of actin filaments, fascin and myosin-II filaments exhibiting pulsatile collective dynamics and superdiffusive transport properties. Both phenomena rely on the complex competition of crosslinking molecules and motor filaments in the network. They are only observed if the relative strength of the binding of myosin-II filaments to the actin network allows exerting high enough forces to unbind actin/fascin crosslinks. This is shown by varying the binding strength of the acto-myosin bond and by combining the experiments with phenomenological simulations based on simple interaction rules.

We have measured the equilibrium constant for the denaturation transition of the engineered fluorescein-binding lipocalin FluA as a function of pressure and temperature, taking advantage of the fact that the ligand's fluorescence is almost fully quenched when complexed with the folded protein, but reversibly reappears on denaturation. From the equilibrium constant as a function of pressure and temperature all of the involved thermodynamic parameters of protein folding, in particular the changes in entropy and volume, compressibility, thermal expansion, and specific heat, were deduced in a global fitting procedure. Assuming that these parameters are independent of temperature and pressure, we can demonstrate from the ratio of ⌬␤, ⌬␣ 2 , ⌬Cp that the phase diagram of protein folding assumes an elliptic shape. Furthermore, we can show that the thermodynamic condition for such an elliptic phase diagram is related to the degree of correlation between the fluctuations of the changes in volume and enthalpy at the phase boundary. For the protein investigated this correlation is low, as generally expected for highly degenerate systems. Our study suggests that the elliptic phase diagram is a consequence of the inherent conformational disorder of proteins and that it may be viewed as the thermodynamic manifestation of the high degeneracy of conformational energies that is characteristic for this class of macromolecules.anticalin ͉ denaturation ͉ engineering ͉ folding ͉ thermodynamics