Animal cells exhibit a wide variety of motile activities, which are essential for the formation and function of human tissues. Actin and its binding proteins provide the essential force-generating machinery for these motilities. Our focus is to investigate how actin binding proteins, tropomyosin and caldesmon, work together in regulating actin filament dynamics and function in nonmuscle cells. We have identified human tropomyosin 5 and a colon epithelial protein as potential autoantigen for human ulcerative colitis. In the process of characterizing tropomyosin isoforms in ulcerative colitis, we have cloned a novel tropomyosin isoform, whose gene product is preferentially expressed in precancerous cells. Actin cytoskeleton (tropomyosin 5) also plays an important role in host cell and parasite, Cryptosporidium parvum, interaction during infection. Molecular Mechanisms of Cardiac Development and Function: Using molecular biology approach, we are defining the cis-regulatory elements and trans-activating factors required for the expression of cardiac troponin T gene. Another approach is to isolate and characterize novel stage- and/or region-specific genes during heart development. We have cloned a novel Xin gene, whose gene product is localized at the intercalated discs in cardiac muscle and the myotendinous junctions in skeletal muscle. In the mouse, there are two Xin genes, mXin· and mXin‚. A mXin· knockout mouse line was generated. The mXin·-deficient mouse hearts exhibit cardiac hypertrophy and cardiomyopathy, due to a disruption of intercalated disc and some of sarcomeres. These results showed that mouse Xin· may play a vital role in cardiac development and function. An upregulation of dystrophin protein was observed in our mXin·-null mouse skeletal muscle, whereas a significant increase in mXin· expression was associated with mdx (a mouse model of human Duchenne Muscular Dystrophy) mouse skeletal muscle. This finding suggests a genetics and functional interaction between mXin· and dystrophin proteins. Currently, generation of a mXin‚ knockout mouse line as well as investigation of mXin· role in skeletal muscle are underway.