Deletion of the myopathy loop of Dictyostelium myosin II and its impact onmotor functions

One of the putative actin-binding sites of Dictyosteliun myosin II is the b eta-strand-turn-beta-strand structure (Ile(398)-Leu-Ala-Gly-Arg-Asp(403)-Le u-Val(405)) the "myopathy loop," which is located at the distal end of the upper 50-kDa subdomain and next to the conserved arginine (Arg(397)), whose mutation in human cardiac myosin results in familial hypertrophic cardiomy opathy. The myopathy loop contains the TEDS residue (Asp(403)), which is a target of the heavy-chain kinase in myosin I, Moreover, the loop contains a cluster of hydrophobic residues (Ile(398), Leu(399), Leu(404), and Val(405 )), whose side chains are fully exposed to the solvent. In our study, the m yopathy loop was deleted from Dictyostelium myosin II to investigate its fu nctional roles. The mutation abolished hydrophobic interactions of actin an d myosin in the strong binding state during the ATPase cycle. Association o f the mutant myosin and actin was maintained only through ionic interaction s under these conditions. Without strong hydrophobic interactions, the muta nt myosin still exhibited motor functions, although at low levels. It is li kely that the observed defects resulted mainly from a loss of the cluster o f hydrophobic residues, since replacement of Asp(403) or Arg(402) with alan ine generated a mutant with less severe or no defects compared with those o f the deletion mutant.