In this paper, the property of the muscle titin protein to form in vitro specific amyloid-like aggregates is discussed. The main difference from the known amyloid aggregates is the formation of a quaternary structure that resembles cross-β, with no changes in the secondary structure. Based on the results obtained earlier, as well as the results of this study, we make assumptions about changes in the structure of titin that occur during the formation of amyloid-like aggregates. In particular, our X-ray diffraction data on the titin aggregates suggest that β-strands in the aggregates of this protein are not located perpendicular to the fibril axis, as described for other amyloid proteins, but in parallel. The distance between the β-sheets in the aggregates may vary, and the β-sheets themselves are not strictly oriented along one of the axes, which can lead to the appearance of a diffuse ring reflection of ~8-12 Å. In this regard, the titin aggregates should not be called amyloid, but amyloid-like, with a quaternary structure that resembles cross-β. It cannot be excluded that the formation of this quaternary structure can also occur due to the partial unfolding of titin domains, followed by the interaction of open β-strands between neighboring domains and/or domains of neighboring molecules.
Keywords: X-ray diffraction; amyloid-like aggregates; cross-β structure; functional amyloids; titin.