Correlation between conformational and binding properties of nebulin repeats

Nebulin, a large protein (600 to 800 kDa) located in the thin filament of striated vertebrate muscle, is assumed to bind and stabilise F-actin. Complete sequence determination of human nebulin has only recently been accomplished showing a uniform modular structure along the whole length of the molecule. Up to 97% of the sequence is assembled from repeats of a sequence motif 35 amino acid residues long. This architecture suggests that a structural and functional understanding of such a large molecule may be possible by characterising single repeats and reconstructing from them the behaviour of the whole molecule. In the present study, we extend and generalise to the whole molecule previous work carried out on single repeats from a limited region of nebulin. Knowledge of the complete sequence allowed extensive analysis of the single repeats revealing a progressive N to C-terminal divergence that is mirrored by an increase of the alpha-helix propensity. A number of synthetic peptides spanning the sequences of selected repeats were obtained and their conformational and binding properties studied in detail. All the peptides showed a tendency to fold as transient helices in aqueous solution with helix content as observed by CD and NMR studies in excellent agreement with predictions. A higher helical tendency of repeats near the C terminus was observed. Analysis of the influence of charged media as well as trifluoroethanol on the folding of single repeats strongly suggested that the mechanism by which the nebulin alpha-helix is stabilised is mostly electrostatic. Peptides with higher helical content also showed a higher binding affinity to F-actin. Considerably varying effects were observed for the peptides on F-actin viscosity and polymerisation. We discuss the divergence in sequence and helical tendency and its correlation to the functional data with regard to their significance for the assembly of the thin filament during myogenesis.