De novo myofibrillogenesis in C2C12 cells: evidence for the independent assembly of M bands and Z disks

We studied the distribution of the giant sarcomeric protein obscurin during de novo myofibrillogenesis in C₂C₁₂ myotubes to learn when it is integrated into developing sarcomeres. Obscurin becomes organized first at the developing M band and later at the mature Z disk. Primordial M bands consisting of obscurin, myomesin, and M band epitopes of titin assemble before adult fast-twitch sarcomeric myosin is organized periodically and nearly concurrently with primitive Z disks, which are composed of -actinin and Z disk epitopes of titin. Z disks and M bands can assemble independently at spatially distant sites. As sarcomerogenesis proceeds, these structures interdigitate to produce a more mature organization. Fast-twitch muscle myosin accumulates in the myoplasm and assembles into A bands only after Z disks and M bands assume their typical interdigitated striations. The periodicities of M bands remain constant at 1.8 µm throughout sarcomerogenesis, whereas distances between Z disks increase from 1.1 µm in early sarcomeres to 1.8 µm in more mature structures. Our findings indicate for the first time that primitive M bands self-assemble independently of Z disks, that obscurin is a component of these primitive M bands in skeletal muscle cells, and that A bands assemble only after M bands and Z disks integrate into maturing sarcomeres. obscurin; titin; myosin; myomesin; -actinin; A band