Skeletal muscle connectin

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... 

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. 

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%. 

Fig. 13. Electron micrograph of myofibrils of frog skeletal muscle treated with polyclonal antibodies against chicken breast muscle /3-connectin. Note that there are several symmetrical stripes in each half of the A bands. For further details, see Maruyama etal. (1985b).

Fig. 14. Diagram of the parallel elastic component of vertebrate skeletal muscle sarcomere. Connectin filaments originating from the edges of the central bare zone of a thick (myosin) filament run through the thin (aclin) filament to the Z lines.

As was already discussed at length above, connectin is a very long, flexible elastic fllament linking the myosin filament to the Z lines in vertebrate skeletal muscle. Therefore, it is very plausible that connectin serves as the parallel elastic component in a myofibril (Fig. 14). The elastic nature of connectin filaments appears to make possible the passive tension generation when a myofibril is stretched beyond the overlap of the thick and thin filaments (Natori, 1954). It also explains why such overstretched myofibrils slowly return to the original state upon release (Natori, 1954). 

Other M. p. are filamin, M, 250,000, which binds ac-tin vinculin, 130,000, which is part of the Z line, and titin (or connectin), M, 250,000, found in cardiac and skeletal muscle. The giant, single molecule of 11-tin forms a filament extending from the M-line to the Z-line in the striated muscle sarcomere. In smooth muscle, a-actinin, vinculin and filamin anchor the thin filaments to the cell membrane. [R.M.Bagby in Newman Stephens (eds.) Biochemistry of Smooth Muscle (CRC Press, Boca Raton, 1983) pp.1-84 R.M.Dowben J.W.Shay (eds.) Cell and Muscle Motility Vol.4 (Plenum, New York, 1983) S.B.Marston C.W.J. Smith J. Muscle Res Cell Motil. 6 (1985) 669-708 K.Wang in Cell and Muscle Motility (ed. J.W.Shay) Vol.6 (Plenum, New York, 1985) pp. 315-369 J.-P.Jin J. Biol. Chem. 270 (1995) 6908-6916 B.J.Agnew etal. J. Biol. Chem. 27 (1995) 17582-17587 A.S.Rovner etal. J. Biol. Chem. 270 (1995) 30260-30263]... 

In chicken tissues, fluorescent antiserum against connectin stained only skeletal and cardiac muscles. Chicken gizzard and aorta were negatively stained (Ikeya et al., 1983). Examinations by SDS-gel electrophoresis confirmed that connectin-like proteins are not present in nonmuscle cells (D. H. Hu, unpublished observations, 1984). 

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