INTRODUCTION Satellite cells, also called adult myoblasts or muscle precursor cells, were first described by Mauro (1961). These mononucleate cells are located between the basement membrane and the plasma membrane of the muscle fiber. A large body of literature has indicated that satellite cells provide myofiber nuclei in the growing muscles. These cells are also activated in injured muscles, where their progeny differentiate and fuse to reconstitute the wounded muscle fibers (for review see Hartley and Yablonka- Reuveni, 1992). In contrast to some commonly used myogenic cell lines such as C2.7, culture of primary satellite cells offers the opportunity to observe complete myogenic differentiation very similar to that occuring in vivo. Myoblasts proliferate, become postmitotic, and differentiate into large multinucleate myotubes which spontaneously contract about 11-12 days after plating (Campion, 1983; Le Moigne et al., 1990). The exit from cell cycling and the differentiation process most likely involve significant alterations of the intracellular protein pool mediated by reprogramming and cytoplasmic site-specific targeting of mRNA expression, and by proteolytic activity. Furthermore, myogenesis involves profound reorganization of the cell structure. Thus, during the fusion of the individual myogenic precursor cells into the syncitial myotubes, the cytoskeletal framework is dramatically modified in relation to the creation of a functional sarcomeric organization. During the course of myogenic differentiation, numerous factors may exert specific control at different key steps. Among the factors acting in myoblast maturation, prosomes appear to be important in view of their multiple biological roles. These particles constitute the proteolytic core of the 26S proteasomes (therefore, also called ‘multicatalytic proteinase’ (MCP) or ‘20S proteasomes’) and also have RNase activities. The 26S proteasomes (‘MCP-complex’ or ‘MCP-C’) were originally found as components of the untranslated mRNA complexes in the cytoplasm and, more recently, as components of pre- mRNPs and the nuclear matrix. They may thus be considered as cellular factors playing an important role in the homeostasis of cellular proteins by protein biosynthesis and breakdown (for review see Coux et al., 1996; Scherrer and Bey, 1994). The prosome particles, first found associated with untranslated mRNPs, may be considered trans-acting factors acting at transcriptional and post-transcriptional levels of gene expression. Prosomes were found on interphase chromosomes, the nuclear matrix and pre-mRNP (Pal et al., 1988; E. Pilotti et al., unpublished), and cytoplasmic mRNP but not in polyribosomes 989 Journal of Cell Science 112, 989-1001 (1999) Printed in Great Britain © The Company of Biologists Limited 1999 JCS9866 Myogenesis proceeds by fusion of proliferating myoblasts into myotubes under the control of various transcription factors. In adult skeletal muscle, myogenic stem cells are represented by the satellite cells which can be cultured and differentiate in vitro. This system was used to investigate the subcellular distribution of a particular type of prosomes at different steps of the myogenic process. Prosomes constitute the MCP core of the 26S proteasomes but were first observed as subcomplexes of the untranslated mRNPs; recently, their RNase activity was discovered. A monoclonal antibody raised against the p27K subunit showed that the p27K subunit-specific prosomes move transiently into the nucleus prior to the onset of myoblast fusion into myotubes; this represents possibly one of the first signs of myoblast switching into the differentiation pathway. Prior to fusion, the prosomes containing the p27K subunit return to the cytoplasm, where they align with the gradually formed lengthwise-running desmin-type intermediate filaments and the microfilaments, co-localizing finally with the actin bundles. The prosomes progressively form discontinuous punctate structures which eventually develop a pseudo- sarcomeric banding pattern. In myotubes just formed in vitro, the formation of this pattern seems to preceed that produced by the muscle-specific sarcomeric α-actin. Interestingly, this pattern of prosomes of myotubes in terminal in vitro differentiation was very similar to that of prosomes observed in vivo in foetal and adult muscle. These observations are discussed in relation to molecular myogenesis and prosome/proteasome function. Key words: Prosome, Proteasome, Cytoskeleton, Satellite cell, Myogenesis, Sarcomere SUMMARY Dynamic distribution and formation of a para-sarcomeric banding pattern of prosomes during myogenic differentiation of satellite cells in vitro J. Foucrier 1, *, M. C. Grand 1 , F. De Conto 2,3 , Y. Bassaglia 1 , G. Géraud 2 , K. Scherrer 2 and I. Martelly 1 1 CRRET, UPRESA-CNRS 7053, Université Paris 12, Av. du Général de Gaulle, 94010 Créteil Cedex, France 2 Institut Jacques Monod, Université Paris 7, 2, place Jussieu, 75251 Paris Cedex 05, France 3 Istituto di Microbiologia, Università degli Studi, Viale A. Gramsci 14, 43100 Parma, Italy *Author for correspondence Accepted 13 January; published on WWW 10 March 1999