Mutations in fast skeletal troponin I, troponin T, and beta-tropomyosin that cause distal arthrogryposis all increase contractile function.
Distal arthrogryposes (DAs) are a group of disorders characterized by congenital contractures of distal limbs without overt neurological or muscle disease. Unexpectedly, mutations in genes encoding the fast skeletal muscle regulatory proteins troponin T (TnT), troponin I (TnI), and beta-tropomyosin (beta-TM) have been shown to cause autosomal dominant DA. We tested how these mutations affect contractile function by comparing wild-type (WT) and mutant proteins in actomyosin ATPase assays and in troponin-replaced rabbit psoas fibers. We have analyzed all four reported mutants: Arg63His TnT, Arg91Gly beta-TM, Arg174Gln TnI, and a TnI truncation mutant (Arg156ter). Thin filaments, reconstituted using actin and WT troponin and beta-TM, activated myosin subfragment-1 ATPase in a calcium-dependent, cooperative manner. Thin filaments containing either a troponin or beta-TM DA mutant produced significantly enhanced ATPase rates at all calcium concentrations without alternating calcium-sensitivity or cooperativity. In troponin-exchanged skinned fibers, each mutant caused a significant increase in Ca2+ sensitivity, and Arg156ter TnI generated significantly higher maximum force. Arg91Gly beta-TM was found to have a lower actin affinity than WT and form a less stable coiled coil. We propose the mutations cause increased contractility of developing fast-twitch skeletal muscles, thus causing muscle contractures and the development of the observed limb deformities.
Robinson
Paul
P
Department of Cardiovascular Medicine, University of Oxford, Oxford OX3 7BN, UK.
Lipscomb
Simon
S
Preston
Laura C
LC
Altin
Elissa
E
Watkins
Hugh
H
Ashley
Christopher C
CC
Redwood
Charles S
CS
D000199:Actins / Q000378:metabolism; D019943:Amino Acid Substitution; D000818:Animals; D001120:Arginine / Q000235:genetics; D001176:Arthrogryposis / Q000235:genetics* / Q000503:physiopathology; D002118:Calcium / Q000378:metabolism; D005998:Glycine / Q000235:genetics; D006801:Humans; D009119:Muscle Contraction / Q000235:genetics* / Q000502:physiology; D018482:Muscle, Skeletal; D009154:Mutation; D009218:Myosins / Q000378:metabolism; D011817:Rabbits; D014335:Tropomyosin / Q000235:genetics*; D019210:Troponin I / Q000235:genetics*; D020107:Troponin T / Q000235:genetics*
D000199:Actins; D014335:Tropomyosin; D019210:Troponin I; D020107:Troponin T; D001120:Arginine; D009218:Myosins; D002118:Calcium; D005998:Glycine