Analyses of recombinant mutant cardiac troponin T in vitro and in myocyte culture
Redwood C., Esposito G., Sweeney HL., Watkins H.
Mutations in six cardiac contractile protein genes cause hypertrophic cardiomyopathy (HCM). Most HCM mutations are missense mutations likely to encode stable peptides that incorporate into the sarcomere but then interfere with function (dominant negatives). However, as HCM can also be caused by truncation mutations which may encode no stable protein (null alleles), alterations of the relative amounts of contractile proteins may also cause cardiac hypertrophy. Wild type and truncated human cardiac tropenin T (TnT) were expressed in a novel quail myotube system. Antibody-staining demonstrated incorporation of both wild-type and truncated TnT into the quail sarcomeres. Ca ++ -activated force of contraction was normal in transfected myotubes expressing wild type human cardiac TnT, but 80% reduced with the truncated TnT. Thus the truncated TnT is not a null allele, but acts as a dominant negative. In view of this, and a recent report of a missense TnT (Ile79Asn) producing increased velocity in the (unloaded) motility assay, we have studied 3 missense TnT peptides that cause HCM. All 3 were stable and incorporated into the sarcomere. The Ile79Asn mutant caused both depression of max. Ca ++ -activated force (by 25%) and a shift of the pCa curve (decreased Ca ++ sensitivity). With increased velocity, but decreased force, we suggest that the mutant TnT acts by shortening the cross-bridge cycle. This is the first indication that the troponin complex can regulate the actin/myosin interaction beyond acting as an on/off switch. To begin to dissect this mechanism we have expressed the mutant TnTs (together with troponins C and I and α-tropomyosin) in E. coli. Analyses of the purified proteins will allow us to examine the TnT-tropomyosin interaction and, by reconstitution, the impact of TnT mutations on the function of the thin filament.