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Xin is a protein that is expressed during early developmental stages of cardiac and skeletal muscles. Immunolocalization studies indicated a peripheral localization in embryonic mouse heart, where Xin localizes with beta- catenin and N-cadherin. In adult tissues, Xin is found primarily in the intercalated discs of cardiomyocytes and the myotendinous junctions of skeletal muscle cells, both specialized attachment sites of the myofibrillar ends to the sarcolemma. A large part of the Xin protein consists of unique 16 amino acid repeats with unknown function. We have investigated the characteristics of the Xin repeats by transfection experiments and actin-binding assays and ascertained that, upon expression in cultured cells, these repeats bind to and stabilize the actin-based cytoskeleton. In vitro co- sedimentation assays with skeletal muscle actin indicated that they not only directly bind actin filaments, but also have the capability of arranging microfilaments into networks that sediment upon low-speed centrifugation. Very similar repeats were also found in Xin-repeat protein 2' (XIRP2), a novel protein that seems to be expressed mainly in striated muscles. Human XIRP2 contains 28 Xin repeats with properties identical to those of Xin. We conclude that the Xin repeats define a novel, repetitive actin-binding motif present in at least two different muscle proteins. These Xin- repeat proteins therefore constitute the first two members of a novel family of actin-binding proteins
Filamin c is the predominantly expressed filamin isoform in striated muscles. It is localized in myofibrillar Z- discs, where it binds FATZ and myotilin, and in myotendinous junctions and intercalated discs. Here, we identify Xin, the protein encoded by the human gene 'cardiomyopathy associated 1' (CMYA1) as filamin c binding partner at these specialized structures where the ends of myofibrils are attached to the sarcolemma. Xin directly binds the EVH1 domain proteins Mena and VASP. In the adult heart, Xin and Mena/VASP colocalize with filamin c in intercalated discs. In cultured cardiomyocytes, the proteins also localize in the nonstriated part of myofibrils, where sarcomeres are assembled and an extensive reorganization of the actin cytoskeleton occurs. Unusual intraexonic splicing events result in the existence of three Xin isoforms that associate differentially with its ligands. The identification of the complex filamin c-Xin-Mena/VASP provides a first glance on the role of Xin in the molecular mechanisms involved in developmental and adaptive remodeling of the actin cytoskeleton during cardiac morphogenesis and sarcomere assembly. (c) 2006 Elsevier Inc. All rights reserved
Context. X-ray radiation from accreting compact objects is an important part of stellar feedback. The metal-poor galaxy ESO 338-4 has experienced vigorous starburst during the last <40 Myr and contains some of the most massive super star clusters in the nearby Universe. Given its starburst age and its star-formation rate, ESO 338-4 is one of the most efficient nearby manufactures of neutron stars and black holes, hence providing an excellent laboratory for feedback studies. Aims. We aim to use X-ray observations with the largest modern X-ray telescopes XMM-Newton and Chandra to unveil the most luminous accreting neutron stars and black holes in ESO 338-4. Methods. We compared X-ray images and spectra with integral field spectroscopic observations in the optical to constrain the nature of strong X-ray emitters. Results. X-ray observations uncover three ultraluminous X-ray sources (ULXs) in ESO 338-4. The brightest among them, ESO 338 X-1, has X-ray luminosity in excess of 10(40) erg s(-1). We speculate that ESO 338-4 X-1 is powered by accretion on an intermediate-mass (greater than or similar to 300 M-circle dot)black hole. We show that X-ray radiation from ULXs and hot superbubbles strongly contributes to He II ionization and general stellar feedback in this template starburst galaxy.