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1 Physiology, University of Maryland, Baltimore, Baltimore, MD, USA
* To whom correspondence should be addressed. E-mail: akons001{at}umaryland.edu.
We studied the distribution of the giant sarcomeric protein, obscurin, during de novo myofibrillogenesis in C2C12 myotubes, to learn when it integrates into developing sarcomeres. Obscurin first becomes organized at the developing M-line and later at the mature Z-disk. Primordial M-lines, consisting of obscurin, myomesin and M-line epitopes of titin, assemble before adult fast twitch sarcomeric myosin is periodically organized and nearly concurrently with primitive Z-disks, composed of 
-actinin and Z-disk epitopes of titin. Z-disks and M-lines can assemble independently, at spatially distant sites. As sarcomerogenesis proceeds, these structures interdigitate to give rise to a more mature organization. Fast-twitch muscle myosin accumulates in the myoplasm and assembles into A-bands only after Z-disks and M-lines assume their typical interdigitated striations. The periodicities of M-lines 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-lines self-assemble independently of Z-disks, that obscurin is a component of these primitive M-lines, and that A-bands only assemble after M-lines and Z-disks integrate into maturing sarcomeres.
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