Isolation and characterization of titin T1 from bovine cardiac muscle

Methods were developed for isolation of bovine cardiac titin in the apparently full-sized T1 form. The pure protein could be obtained either in the denatured state after chromatography of myofibril extracts on Sephacryl S-1000 or in the native state using hydroxylapatite chromatography. Titin-specific chicken polyclonal antibodies against denatured titin stained the I band regions and weakly stained the M line area of cardiac myofibrils but primarily stained the A-I junction positions in skeletal muscle myofibrils. Bovine cardiac titin contained 176 +/- 5 mumol of sulfhydryl groups per gram in the presence of 6 M guanidine hydrochloride or 0.1% SDS; only 45% of these groups were reactive under non-denaturing condition. Circular dichroism measurements indicated that titin contained approximately 53% beta-sheet and 47% random coil at pH 7.0. These results are consistent with the idea that titin is primarily composed of immunoglobulin C2 and fibronectin type III like motifs throughout its length, including the flexible I band region.