Many of the functional domains of the myosin molecule have been defined by the use of proteolytic enzymes. Major fragments that retain enzymatic or assembly properties have been prepared by cleavage in the rod to form heavy meromyosin (HMM) and light meromyosin (LMM)1 or at the head-rod junction to form S-1 and rod2,3. Limited tryptic digestion of vertebrate skeletal myosin S-14 indicates that the head contains three major regions: an ammo-terminal nucleotide binding domain of molecular weight (MW) 25,0005,6, a central domain of MW 50,000 and a carboxyl domain MW 20,000; the latter two are both able to bind to actin7,8. Tryptic digestion of scallop S-1 has also been used to isolate a head fragment MW 14,000 associated with both types of scallop light chains9. Here we report that myosin from vertebrate (chicken and rabbit skeletal) and molluscan (scallop adductor) striated muscles is cleaved in an unusual way with an enzyme from Pseudomonas aeruginosa10-12. This bacterial protease (designated Ps-1) does not cleave myosin at the head-rod junction or in the rod; instead, Ps-1 splits the myosin heavy chain within the head, yielding a complete rod joined to the 20,000-MW head domains. The scallop regulatory and essential light chains remain associated with this fragment. We examined this new fragment by electron microscopy; the rods bear two 'nubs' about 100 Aring; long, which appear to correspond morphologically to the neck region of the myosin molecule13,14.
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