Immunocytochemical studies using a monoclonal antibody to bovine cardiac titin on intact and extracted myofibrils.

A monoclonal antibody specific to bovine cardiac titin has been identified. The antibody recognizes a common antigenic site in striated muscles of several species. In relaxed myofibrils, specific staining at the A-I junction resulted in a doublet of fluorescent bands within a sarcomere. The distance between the doublets in successive sarcomeres varied according to the degree of myofibrillar contraction. Staining on formamide-extracted myofibrils has confirmed that this epitope is located near the outer edges of isolated A bands. Selective extraction of myofibrillar proteins resulted in different staining patterns. Disrupting the structural integrity of the M-line or the A-band centre caused a significant amount of titin to translocate toward the Z-line region. In contrast, shortening of the A-band by removal of myosin from the ends of the thick filaments resulted in anti-titin staining moving closer to the M-line region. Several conclusions can be drawn from this study: (a) two aligned groups of titin molecules are placed symmetrically to the M-line in a sarcomere; (b) titin may attach directly or via intermediary protein(s) to sites near the M-line and Z-line such that the protein is under tension and (c) removal of proteins from either region results in titin staining in the opposite region. However, the edges of the A-band give some hindrance to collapse of the titin toward the M-line.