The Drosophila troponin-tropomyosin complex and regulation of muscle contraction. J.C. Sparrow ¹, U. Nongthomba ¹, S. Clark ¹, S. Haigh ¹, a. Ferrus ². 1) Dept Biol, Univ York, York, YO10 5DD, England; 2) Institut Cajal, c\ Dr Arce 37, Madrid 28002, Spain.

   We are investigating the molecular interactions involved in muscle thin filament regulation using the indirect flight muscles (IFMs). We have recovered 17 suppressors of the hdp² (TnI gene) and up¹⁰¹ (TnT gene) mutations which disrupt thin filament regulation and cause IFM hypercontraction.
    The hdp² mutation (A116V) and an intragenic suppressor (R113C) lie within a TnI-TnC contact site; modelling suggests the suppressor compensates for the increased residue size of hdp². Three suppressors are myosin heavy chain (Mhc) mutations. Since a myosin transgene lacking a myosin head suppresses hdp² hypercontraction the Mhc suppressors probably reduce force production, so preventing hypercontraction.
    The D53 suppressor mutation is S185F in the Tm2 gene. Residue S185 is close to or within the calcium-sensitive Tm-TnT binding site. This substitution may affect the interaction of TnT and Tm, or the ability of the Tm to traverse the F-actin surface. Two selected suppressors are in the Act88F IFM-specific actin gene. We have since determined that 4 other Act88F mutants suppress hypercontraction. One, Act88F-E57K, gives full suppression and allows flight.
    hdp² affects the speed of all fly and larval behaviours. Only in the IFMs does it produce hypercontraction. Clearly in non-IFM muscles regulation of contraction is almost normal. hdp² may makes the system more sensitive to Ca²⁺; or more difficult to re-establish the relaxed (inhibited) state. We are investigating which is correct by expressing the proteins in E. coli to reconstitute thin filaments for biochemical analyses.