ISOFORM-SPECIFIC INTERACTION OF TROPOMODULIN WITH SKELETAL-MUSCLE ANDERYTHROCYTE TROPOMYOSINS

Tropomodulin is a tropomyosin-binding protein that localizes to the po inted end of striated muscle thin filaments and caps the pointed end o f tropomyosin-actin filaments in vitro. Results from previous studies have suggested that tropomyosin-tropomodulin interactions are isoform- specific. To investigate the molecular basis for the isoform specific interactions of tropomyosin and tropomodulin, we isolated a cDNA for c hicken skeletal muscle tropomodulin. The derived amino acid sequence o f muscle tropomodulin is 86% identical with that of human erythrocyte tropomodulin, indicating that tropomodulins are highly conserved prote ins. Multiple mRNAS seen on Northern blots of chicken skeletal muscle mRNA can be accounted for by multiple polyadenylation signals in the 3 '-untranslated region of the cDNA. I-125-Labeled skeletal muscle and e rythrocyte tropomyosins were assayed for binding bacterially expressed muscle tropomodulin using a solid phase binding assay. Unexpectedly, skeletal muscle and erythrocyte tropomyosins bound with similar affini ties to muscle tropomodulin (K-d values approximately 0.2 mu M). Howev er, cross-competition studies using erythrocyte and skeletal muscle tr opomyosins indicated that they bound different sites on tropomodulin. Competition studies using recombinant fragments of tropomodulin to map the binding domains for the two tropomyosins demonstrated that residu es 6-94 contained the skeletal muscle tropomyosin binding site and res idues 90-184 contained the erythrocyte tropomyosin binding site. Bindi ng of different regions of tropomodulin to muscle and non-muscle tropo myosins may permit interaction of tropomodulin with tissue-specific co mponents or may influence the pointed end capping activity of tropomod ulin.