Identification of an N2 line protein of striated muscle.

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Abstract
Three unusually large myofibrillar proteins (MW > 500,000) are present in major amounts in the striated muscle of various species. The molecular properties and localization of 2 of these proteins (MW .apprxeq. 1 .times. 106), named "titin," have been reported recently. The 3rd protein (MW .apprxeq. 5-6 .times. 105), designated "band 3," was purified from rabbit psoas myofibrils. Band 3 is distinct chemically and immunochemically from titin and myosin heavy chain. Immunofluorescent localization studies of band 3 in glycerinated rabbit psoas myofibrils indicated a biphasic distribution. In sarcomeres 1.8-3.7 .mu.m long, band 3 is localized predominately on the N2 line in the phase-contrast lucent region of the I band. Quantitative analysis of light micrographs confirmed the unique behavior of N2 lines described in early EM studies. As the sarcomeres lengthen, the N2 line moves away from both the Z line and the M line, maintaining the same proportional distance. In very short sarcomeres (1.5-1.8 .mu.m), band 3 is found mainly in the A band, suggesting that either the N2 line moves into the A band or that band 3 has dissociated from the N2 line which remains near the Z line. Band 3 is probably an N2 line component of rabbit skeletal myofibrils.