TYPE-2X-MYOSIN HEAVY-CHAIN IS CODED BY A MUSCLE-FIBER TYPE-SPECIFIC AND DEVELOPMENTALLY-REGULATED GENE

We have previously reported the identification of a distinct myosin he avy chain (MyHC) isoform in a major subpopulation of rat skeletal musc le fibers, referred to as 2X fibers (Schiaffino, S., L. Gorza, S. Sart ore, L. Saggin, M. Vianello, K. Gundersen, and T. Lomo. 1989. J. Muscl e Res. Cell Motil. 10:197-205). However, it was not known whether 2X-M yHC is the product of posttranslational modification of other MyHCs or is coded by a distinct mRNA. We report here the isolation and charact erization of cDNAs coding a MyHC isoform that is expressed in type 2X skeletal muscle fibers. 2X-MyHC transcripts differ from other MyHC tra nscripts in their restriction map and 3' end sequence and are thus der ived from a distinct gene. In situ hybridization analyses show that 2X -MyHC transcripts are expressed at high levels in the diaphragm and fa st hindlimb muscles and can be coexpressed either with 2B- or 2A-MyHC transcripts in a number of fibers. At the single fiber level the distr ibution of each MyHC mRNA closely matches that of the corresponding pr otein, determined by specific antibodies on serial sections. In hindli mb muscles 2X-, 2A-, and 2B-MyHC transcripts are first detected by pos tnatal day 2-5 and display from the earliest stages a distinct pattern of distribution in different muscles and different fibers. The emerge nce of type 2 MyHC isoforms thus defines a distinct neonatal phase of fiber type differentiation during muscle development. The functional s ignificance of MyHC isoforms is discussed with particular reference to the velocity of shortening of skeletal muscle fibers.