Connectin

Maruyama, 1997] Maruyama, K. Connectin/titin, a giant elastic protein of muscle. FASEB J. 11 (1997) 341-345... 

In addition to actin and myosin, other proteins are found in the two sets of filaments. Tropomyosin and a complex of three subunits collectively called troponin are present in the thin filaments and play an important role in the regulation of muscle contraction. Although the proteins constituting the M and the Z bands have not been fully characterized, they include a-actinin and desmin as well as the enzyme creatine kinase, together with other proteins. A continuous elastic network of proteins, such as connectin, surround the actin and myosin filaments, providing muscle with a parallel passive elastic element. Actin forms the backbone of the thin filaments [4]. The thin... 

Wang K. Titin/connectin and nebulin giant protein rulers of muscle structure and function. Adv Biophys 1996 33 123-134. 

Kinbara, K., Ishiura, S., Tomioka, S., Sorimachi, H., Jeong, S.Y., Amano, S., Kawasaki, H., Kolmerer, B., Kimura, S., Labeit, S., Suzuki, K., 1998, Purification of native p94, a muscle-specific calpain, and characterization of its autolysis, Biochem J., 335, 589-96 Kinbara, K., Sorimachi, H., Ishiura, S., Suzuki, K., 1997, Muscle-specific calpain, p94, interacts with the extreme C-terminal region of connectin, a unique region flanked by two immunoglobulin C2 motifs, Arch. Biochem. Biophys, 342, 99-107... 

Sorimachi, H., Ono, Y., Suzuki, K., 2000, Skeletal muscle-specific calpain, p94, and connectin/titin their physiological junctions and relationship to limb-girdle muscular dystrophy type 2A, Adv. Exp. Med. Biol., 481, 383-95... 

Higuchi, H., Nakauchi, Y., and Maruyama, K. (1993). Characterization of beta-connectin (titin 2) from striated muscle by dynamic light scattering. Biophys. J. 65, 1906-1915. 

Hu, D. H., Kimura, S., and Maruyama, K. (1986). Sodium dodecyl sulfate gel electrophoresis studies of connectin-like high molecular weight proteins of various types of vertebrate and invertebrate muscles./. Biochem. (Tokyo) 99, 1485—1492. 

Itoh, Y., Suzuki, T., and Kimura, S. (1988). Extensible and less-extensible domains of connectin filaments in stretched vertebrate skeletal muscle sarcomeres as detected by immunofluorescence and immunoelectron microscopy using monoclonal antibodies. / Biochem. (Tokyo) 104, 504-508. 

Maruyama, K. (1976). Connectin, an elastic protein from myofibrils. / Biochem. (Tokyo) 80, 405-407. 

Sorimachi, H., Kinbara, K., and Kimura, S. (1995). Muscle-specific calpain, p94, responsible for limb girdle muscular dystrophy type 2A, associates with connectin through IS2, a p94-specific sequence./. Biol. Chem. 270, 31158-31162. 

Tatsumi, R., Maeda, K., and Hattori, A. (2001). Calcium binding to an elastic portion of connectin/titin filaments. / Muscle Res. Cell Motil. 22, 149-162. 

The thick filament system, which comprises myosin protein, connected from the M-line to the Z-disc by titin (connectin), and myosin-binding protein C, which binds at one end to the thick filament and at the other to actin. 

Connectin (titin) 2800 (2100) 10 A-1 Links myosin filament to Z line... 

In striated muscle, the main cytoskeletal proteins are connectin and nebulin, which together constitute 15% of the total myofibrillar proteins (Table I). The two proteins will be discussed in Section III. 

In this article, we use the term connectin instead of titin. 

Connectin consists of doublet bands, a- and j8-connectins (also called titin 1 and titin 2), and it appears that /3-connectin is a proteolytic product of a-connectin (see Section III,E). j8-Connectin is extracted, together with myosin, in a salt solution (Maruyama et al., 1981a), leaving a-con-nectin in the residues. When actin is solubilized with 0.6 M KI, the remaining a- and j3-connectins are translocated toward the Z lines and become insoluble together with residual myosin and actin (Granger and Lazarides, 1978). 

Connecdn content in myofibrils of vertebrate skeletal muscle is as high as 10% of the total myofibrillar structural proteins. Since the connectin content is next in amount to that of actin, Wang et al. (1979) called connectin the third most abundant protein of muscle. Seki and Wa-tanabe (1984) confirmed that the connectin content of carp skeletal muscle was 13% by measuring the amount of the gel-filtered connectin. Direct estimation of the connectin content in cow semimembranous muscle showed a value of 12% (King, 1984). Trinick et al. (1984) also reported the connectin content in rabbit psoas muscle to be approximately 10%. 

Estimation of the molecular weight of such a giant molecule as connectin is a very difficult problem. Using cross-linked myosin heavy chains (MHC) as standard, Wang (1982) estimated the molecular weight as 1.2 and 1.4 million, respectively, for a- and /3-connectins. In this study it was assumed that MHC are cross-linked to form dimer, trimer, tetramer, etc. However, it turned out that MHC first dimerized and then the dimers... 

Connectin is soluble at more than 0.1 M KCl at natural pH. At lower salt concentrations it forms aggregates. Connectin is precipitated by (NH4)aS04 at 35% saturation, but it is necessary to dialyze against 0.6 M KCl, pH 7.9, to redissolve the precipitate (Trinick et al, 1984). 

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