Conformational Studies of a Two-Module Fragment of Nebulin and Implications for Actin Association

Nebulin, a family of giant myofibrillar proteins of 700-900 kDa, has been proposed as a length-regulating template for the thin filaments of skeletal muscle. In the present study, the conformational states of a two-module nebulin fragment (ND8) were investigated by circular dichroism, steady-state fluorescence, fluorescence lifetime, and one-dimensional and two-dimensional NMR techniques. We observed the following: (a) Contrary to the predicted high α-helical content by computer analysis, CD spectra of ND8 indicated only a maximum of 10% α-helix and 25% β-sheet in a variety of buffers; (b) The presence of increasing concentrations of trifluoroethanol (TFE) promoted the formation of α-helix and ND8 contained 50% α-helix at 67% TFE; (c) Measurement of fluorescence lifetime, anisotropy, and CD of tyrosines in ND8 in various concentrations of TFE indicates that tyrosines are incorporated into the newly formed helical segments; (d) Preliminary correlation spectroscopy and nuclear Overhauser enhancement spectroscopy NMR spectra of ND8 confirmed the formation of α-helical structure and identified sites of helical segments in 30% TFE around the highly conserved tyrosine residues. These data suggest that the conformation of nebulin and its mode of interaction with actin are distinct from the helical tropomyosin.