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Nucleotide excision repair (NER) is carried out by xeroderma pigmentosum (XP) factors. Before the excision reaction, DNA damage is recognized by a complex originally thought to contain the XP group C responsible gene product (XPC) and the human homologue of Rad23 B (HR23B). Here, we show that centrin 2/caltractin 1 (CEN2) is also a component of the XPC repair complex. We demonstrate that nearly all XPC complexes contain CEN2, that CEN2 interacts directly with XPC, and that CEN2, in cooperation with HR23B, stabilizes XPC, which stimulates XPC NER activity in vitro. CEN2 has been shown to play an important role in centrosome duplication. Thus, those findings suggest that the XPC-CEN2 interaction may reflect coupling of cell division and NER.

We have tested the hypothesis that kinesin-1A (formerly KIF5A) is an anterograde motor for axonal neurofilaments. In cultured sympathetic neurons from kinesin-1A knockout mice, we observed a 75% reduction in the frequency of both anterograde and retrograde neurofilament movement. This transport defect could be rescued by kinesin-1A, and with successively decreasing efficacy by kinesin-1B and kinesin-1C. In wild-type neurons, headless mutants of kinesin-1A and kinesin-1C inhibited both anterograde and retrograde movement in a dominant-negative manner. Because dynein is thought to be the retrograde motor for axonal neurofilaments, we investigated the effect of dynein inhibition on anterograde and retrograde neurofilament transport. Disruption of dynein function by using RNA interference, dominantnegative approaches, or a function-blocking antibody also inhibited both anterograde and retrograde neurofilament movement. These data suggest that kinesin-1A is the principal but not exclusive anterograde motor for neurofilaments in these neurons, that there may be some functional redundancy among the kinesin-1 isoforms with respect to neurofilament transport, and that the activities of the anterograde and retrograde neurofilament motors are tightly coordinated. INTRODUCTIONStudies on cultured neurons using live-cell fluorescence imaging have demonstrated that neurofilament polymers move rapidly and intermittently along axons in both anterograde and retrograde directions (Roy et al., 2000;Wang et al., 2000;Wang and Brown, 2001;Uchida and Brown, 2004;Yan and Brown, 2005). The instantaneous rate of movement is fast, with peak velocities of up to 3.5 m/s, but the overall rate is slow because the movements are interrupted by prolonged pauses (Brown, 2000(Brown, , 2003Brown et al., 2005;Trivedi et al., 2007). The rapid movement of neurofilaments in axons indicates that these cytoskeletal polymers are transported by fast motors, but the identity of these motors and the mechanism of movement are not well understood.Several lines of evidence suggest that neurofilaments move along microtubule tracks, propelled by microtubule motor proteins (Prahlad et al., 2000;Shah et al., 2000;Francis et al., 2005), and that dynein/dynactin is the retrograde motor (Shah et al., 2000;Helfand et al., 2003;Wagner et al., 2004;He et al., 2005): 1) dynein and dynactin copurify with neurofilaments (Shah et al., 2000) and dynein/dynactin interacts with neurofilaments, possibly via an interaction between neurofilament protein M (NFM) and the dynein intermediate chain (Wagner et al., 2004); 2) knockdown of dynein heavy chain in cultured rat sympathetic neurons blocks retrograde neurofilament movement (He et al., 2005); and 3) dynein has also been shown to colocalize with peripherin in PC12 cells and overexpression of p50/dynamitin in those cells leads to a redistribution of peripherin to the distal regions of neurites, consistent with a selective inhibition of retrograde transport (Helfand et al., 2003).Less is known about the anterograde motor for neuro...

A protein complex which specifically complements defects of XP-C cell extracts in vitro was previously purified to near homogeneity from HeLa cells. The complex consists of two tightly associated proteins: the XPC gene product and HHR23B, one of two human homologs of the Saccharomyces cerevisiae repair gene product Rad23 (Masutani et al., EMBO J. 13:1831-1843, 1994). To elucidate the roles of these proteins in "genomeoverall" repair, we expressed the XPC protein in a baculovirus system and purified it to near homogeneity. The recombinant human XPC (rhXPC) protein exhibited a high level of affinity for single-stranded DNA and corrected the repair defect in XP-C whole-cell extracts without extra addition of recombinant HHR23B (rHHR23B) protein. However, Western blot (immunoblot) experiments revealed that XP-C cell extracts contained excess endogenous HHR23B protein, which might be able to form a complex upon addition of the rhXPC protein. To investigate the role of HHR23B, we fractionated the XP-C cell extracts and constructed a reconstituted system in which neither endogenous XPC nor HHR23B proteins were present. In this assay system, rhXPC alone weakly corrected the repair defect, while significant enhancement of the correcting activity was observed upon coaddition of rHHR23B protein. Stimulation of XPC by HHR23B was found with simian virus 40 minichromosomes as well as with naked plasmid DNA and with UV-as well as N-acetoxy-2-acetylfluorene-induced DNA lesions, indicating a general role of HHR23B in XPC functioning in the genomeoverall nucleotide excision repair subpathway.

Strong geometrical frustration in magnets leads to exotic states such as spin liquids, spin supersolids, and complex magnetic textures. SrCu 2 ðBO 3 Þ 2 , a spin-1∕2 Heisenberg antiferromagnet in the archetypical Shastry-Sutherland lattice, exhibits a rich spectrum of magnetization plateaus and stripe-like magnetic textures in applied fields. The structure of these plateaus is still highly controversial due to the intrinsic complexity associated with frustration and competing length scales. We discover magnetic textures in SrCu 2 ðBO 3 Þ 2 via magnetostriction and magnetocaloric measurements in fields up to 100.75 T. In addition to observing low-field fine structure with unprecedented resolution, the data also reveal lattice responses at 73.6 Tand at 82 T that we attribute, using a controlled density matrix renormalization group approach, to a unanticipated 2∕5 plateau and to the long-predicted 1∕2 plateau.high magnetic fields | quantum magnet | field-induced magnetic texturing | density matrix renormalization group | magnetocaloric effect Q uantum paramagnets are Mott insulators where dominant intracell antiferromagnetic interactions lead to a singlet ground state that remains stable for small intercell interactions. The low energy spectrum of triplet excitations is gapped. For systems with uniaxial symmetry, the gap (Δ) is closed by applying a magnetic field gμ 0 H 0 ≈ Δ along the symmetry c-axis (see Fig. 1 A and B). The low-energy degrees of freedom can be mapped onto a gas of hard-core bosons (1). In this description, the external magnetic field plays the role of a chemical potential, and the longitudinal magnetization corresponds to the boson concentration. In systems such as BaCuSi 4 O 6 (2), where Cu atoms are arranged in parallel dimers on a square lattice, the triplets condense in a phase-coherent fluid analogous to a Bose-Einstein condensate. On the other hand, if the kinetic energy is highly frustrated, the repulsion between triplets becomes dominant and leads to crystals with superstructures that are highly sensitive to the concentration of triplets or magnetization (3). These crystalline states correspond to Ising-like orderings-i.e., states with spontaneous modulation of the spin component parallel to the field. The orthogonal-dimer geometry of the Shastry-Sutherland lattice (Fig. 1A) (4) and the ratio of next-nearest to nearest neighbor exchange interactions between the spin-1∕2 Cu 2þ ions, with J 1 ∕J 0 at approximately 0.62 (J 0 ≈ 74 K), make SrCu 2 ðBO 3 Þ 2 a paradigm of frustrated quantum magnetism (5-8). Nuclear magnetic resonance (9) and torque magnetometry (10, 11) measurements have been used in the past to study the magnetic superstructures in SrCu 2 ðBO 3 Þ 2 , but the strength of required magnetic fields has prevented the unambiguous observation of magnetization fractions beyond 1∕3 of saturation. Sensitive measurements for higher concentrations are crucial for model validation. Indeed, given the limited lattice sizes for which the minimal model for SrCu 2 ðBO 3 Þ 2 can be solved under...