Dynamic light scattering and atomic force microscopy imaging on fragments of β-connectin from human cardiac muscle

In order to investigate the protein folding-unfolding process, dynamic light scattering (DLS) and atomic force microscopy (AFM) imaging were used to study two fragments of the muscle cardiac protein beta-connectin, also known as titin. Both fragments belong to the I band of the sarcomer, and they are composed of four domains from I-27 to I-30 (tetramer) and eight domains from I-27 to I-34 (octamer). DLS measurements provide the size of both fragments as a function of temperature from 20 up to 86 degrees C, and show a thermal denaturation due to temperature increase. AFM imaging of both fragments in the native state reveals a homogeneous and uniform distribution of comparable structures. The DLS and AFM techniques turn out to be complementary for size measurements of the fragments and fragment aggregates. An unexpected result is that the octamer folds into a smaller structure than the tetramer and the unfolded octamer is also smaller than the unfolded tetramer. This feature seems related to the significance of the hydrophobic interactions between domains of the fragment. The longer the fragment, the more easily the hydrophobic parts of the domains interact with each other. The fragment aggregation behavior, in particular conditions, is also revealed by both DLS and AFM as a process that is parallel to the folding-unfolding transition.