Comparison of the Early Stages of Forced Unfolding for Fibronectin Type III Modules

• Published in: Proceedings of the National Academy of Sciences - PNAS 2001-05, Vol.98 (10), p.5590-5595
• Main Authors: Craig, David, Krammer, André, Schulten, Klaus, Vogel, Viola
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Atoms; Biological Sciences; Fibronectins - chemistry; Humans; Hydrogen; Hydrogen bonds; Kinetics; Mechanical forces; Molecular biology; Molecular dynamics; Molecular Sequence Data; Physics; Physiological regulation; Protein Denaturation; Protein refolding; Sequence alignment; Sequence Homology, Amino Acid; Solvation
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.101582198

The structural changes accompanying stretch-induced early unfolding events were investigated for the four type III fibronectin (FN-III) modules, FN-III7, FN-III8, FN-III9, and FN-III10 by using steered molecular dynamics. Simulations revealed that two main energy barriers, I and II, have to be overcome to initiate unraveling of FN-III's tertiary structure. In crossing the first barrier, the two opposing β-sheets of FN-III are rotated against each other such that the β-strands of both β-sheets align parallel to the force vector (aligned state). All further events in the unfolding pathway proceed from this intermediate state. A second energy barrier has to be overcome to break the first major cluster of hydrogen bonds between adjacent β-strands. Simulations revealed that the height of barrier I varied significantly among the four modules studied, being largest for FN-III7 and lowest for FN-III10, whereas the height of barrier II showed little variation. Key residues affecting the mechanical stability of FN-III modules were identified. These results suggest that FN-III modules can be prestretched into an intermediate state with only minor changes to their tertiary structures. FN-III10, for example, extends 12 Å from the native "twisted" to the intermediate aligned state, and an additional 10 Å from the aligned state to further unfolding where the first β-strand is peeled away. The implications of the existence of intermediate states regarding the elasticity of fibrillar fibers and the stretch-induced exposure of cryptic sites are discussed.