FUNCTIONAL RECOVERY OF TROPONIN-I IN A DROSOPHILA HELDUP MUTANT AFTERA 2ND SITE MUTATION

To identify proteins that interact in vivo with muscle components we h ave used a genetic approach based on the isolation of suppressors of m utant alleles of known muscle components. We have applied this system to the case of troponin I (TnI) in Drosophila and its mutant allele he ldup(2) (hdp(2)). This mutation causes an alanine to valine substituti on at position 116 after a single nucleotide change in a constitutive exon. Among the isolated suppressors, one of them results from a secon d site mutation at the TnI gene itself. Muscles endowed with TnI mutat ed at both sites support nearly normal myofibrillar structure, perform notably well in wing beating and flight tests, and isolated muscle fi bers produce active force. We show that the structural and functional recovery in this suppressor does not result from a change in the stoic hiometric ratio of TnI isoforms. The second site suppression is due to a leucine to phenylalanine change within a heptameric leucine string motif adjacent to the actin binding domain of TnI. These data evidence a structural and functional role for the heptameric leucine string th at is most noticeable, if not specific, in the indirect flight muscle.