The three-dimensional structure of the central region of the Z disk of honeybee flight muscle has been determined to a resolution of 70 A by three-dimensional reconstruction from electron micrographs of tilted thin sections. The reconstructions show a complex assembly in which actin filaments terminate and are cross-linked together; a number of structural domains of this network are resolved in quantitative three-dimensional detail. The central region of the Z disk contains two sets of overlapping actin filaments of opposite polarity, which originate in the sarcomeres adjacent to the Z disk, and connections between these filaments. The filaments are deflected by the attachment of cross-links; spacing between filaments change by greater than 100 A during their passage through the Z disk. Each actin filament is linked by connecting structures to four filaments of opposite polarity and two filaments are of the same polarity. Four types of connecting density domain are observed in association with pairs of filaments of opposite polarity: C1, C2, C3, and C5. Two of these, C3 and C5, are associated with the ends of actin filaments. Another connection, C4, is associated with three filaments of the same polarity; C4 is threefold symmetric.

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