Аннотация:A giant multidomain protein of striated and smooth vertebrate muscles, titin, consistsof tandems of immunoglobulin (Ig)- and fibronectin type III (FnIII)-like domains representing -sandwiches, as well as of disordered segments. Chicken smooth muscles express several titin isoformsof ~500–1500 kDa. Using various structural-analysis methods, we investigated in vitro nonspecificamyloid aggregation of the high-molecular-weight isoform of chicken smooth-muscle titin (SMTHMW,~1500 kDa). As confirmed by X-ray diffraction analysis, under near-physiological conditions, theprotein formed amorphous amyloid aggregates with a quaternary cross- structure within a relativelyshort time (~60 min). As shown by circular dichroism and Fourier-transform infrared spectroscopy,the quaternary cross- structure—unlike other amyloidogenic proteins—formed without changes inthe SMTHMW secondary structure. SMTHMW aggregates partially disaggregated upon increasing theionic strength above the physiological level. Based on the data obtained, it is not the complete proteinbut its particular domains/segments that are likely involved in the formation of intermolecularinteractions during SMTHMW amyloid aggregation. The discovered properties of titin position thisprotein as an object of interest for studying amyloid aggregation in vitro and expanding our views ofthe fundamentals of amyloidogenesis.