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      Titin-Isoform Dependence of Titin-Actin Interaction and Its Regulation by S100A1/Ca 2+ in Skinned Myocardium

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          Abstract

          Titin, also known as connectin, is a large filamentous protein that greatly contributes to passive myocardial stiffness. In vitro evidence suggests that one of titin's spring elements, the PEVK, interacts with actin and that this adds a viscous component to passive stiffness. Differential splicing of titin gives rise to the stiff N2B and more compliant N2BA isoforms. Here we studied the titin-isoform dependence of titin-actin interaction and studied the bovine left atrium (BLA) that expresses mainly N2BA titin, and the bovine left ventricle (BLV) that expresses a mixture of both N2B and N2BA isforms. For comparison we also studied mouse left ventricular (MLV) myocardium which expresses predominately N2B titin. Using the actin-severing protein gelsolin, we obtained evidence that titin-actin interaction contributes significantly to passive myocardial stiffness in all tissue types, but most in MLV, least in BLA, and an intermediate level in BLV. We also studied whether titin-actin interaction is regulated by S100A1/calcium and found that calcium alone or S100A1 alone did not alter passive stiffness, but that combined they significantly lowered stiffness. We propose that titin-actin interaction is a “viscous break” that is on during diastole and off during systole.

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          Most cited references47

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          The structure and function of proline-rich regions in proteins.

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            Atomic structure of the actin:DNase I complex.

            The atomic models of the complex between rabbit skeletal muscle actin and bovine pancreatic deoxyribonuclease I both in the ATP and ADP forms have been determined by X-ray analysis at an effective resolution of 2.8 A and 3A, respectively. The two structures are very similar. The actin molecule consists of two domains which can be further subdivided into two subdomains. ADP or ATP is located in the cleft between the domains with a calcium ion bound to the beta- or beta- and gamma-phosphates, respectively. The motif of a five-stranded beta sheet consisting of a beta meander and a right handed beta alpha beta unit appears in each domain suggesting that gene duplication might have occurred. These sheets have the same topology as that found in hexokinase.
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              The complete gene sequence of titin, expression of an unusual approximately 700-kDa titin isoform, and its interaction with obscurin identify a novel Z-line to I-band linking system.

              Titin is a giant vertebrate striated muscle protein with critical importance for myofibril elasticity and structural integrity. We show here that the complete sequence of the human titin gene contains 363 exons, which together code for 38 138 residues (4200 kDa). In its central I-band region, 47 novel PEVK exons were found, which contribute to titin's extensible spring properties. Additionally, 3 unique I-band titin exons were identified (named novex-1 to -3). Novex-3 functions as an alternative titin C-terminus. The novex-3 titin isoform is approximately 700 kDa in size and spans from Z1-Z2 (titin's N-terminus) to novex-3 (C-terminal exon). Novex-3 titin specifically interacts with obscurin, a 721-kDa myofibrillar protein composed of 57 Ig/FN3 domains, followed by one IQ, SH3, DH, and a PH domain at its C-terminus. The obscurin domains Ig48/Ig49 bind to novex-3 titin and target to the Z-line region when expressed as a GFP fusion protein in live cardiac myocytes. Immunoelectron microscopy detected the C-terminal Ig48/Ig49 obscurin epitope near the Z-line edge. The distance from the Z-line varied with sarcomere length, suggesting that the novex-3 titin/obscurin complex forms an elastic Z-disc to I-band linking system. This system could link together calcium-dependent, SH3-, and GTPase-regulated signaling pathways in close proximity to the Z-disc, a structure increasingly implicated in the restructuring of sarcomeres during cardiomyopathies.
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                Author and article information

                Journal
                J Biomed Biotechnol
                JBB
                Journal of Biomedicine and Biotechnology
                Hindawi Publishing Corporation
                1110-7243
                1110-7251
                2010
                14 April 2010
                : 2010
                : 727239
                Affiliations
                1Department of Physiology, Sarver Molecular Cardiovascular Research Program, University of Arizona, P.O. Box 245217, Tucson, AZ 85724, USA
                2Department of Food Science and Technology, National Fisheries University, 2-7-1 Nagata-honmachi, Shimonoseki, Yamaguchi 759-6595, Japan
                Author notes

                Academic Editor: Aikaterini Kontrogianni-Konstantopoulos

                Article
                10.1155/2010/727239
                2855102
                20414336
                d246addc-a84f-488e-bb12-4904b13ce40b
                Copyright © 2010 Hideto Fukushima et al.

                This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

                History
                : 6 December 2009
                : 15 January 2010
                Categories
                Research Article

                Molecular medicine
                Molecular medicine

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