2 FUNCTIONALLY DISTINCT MYOSIN HEAVY-CHAIN ISOFORMS IN SLOW SKELETAL-MUSCLE FIBERS

The head part of the myosin heavy chain (MHC) represents the essential component of the molecular force-generating system of muscle [1-3]. T o date, three fast but only one slow MHC isoforms have been identified in adult mammalian limb muscles [4,5]. We show here two functionally different slow MHC isoforms, MHCI beta and MHCIa, coexisting in a cons iderable fraction of slow fibres of rabbit plantaris muscle. The two i soforms exhibit distinct electrophoretic mobilities and different kine tic properties, Thus, as it is known for the fast muscle, also the slo w muscle seems to use different MHC isoforms in order to fulfil differ ent functional demands. (C) 1997 Federation of European Biochemical So cieties.