Redox regulation of protein nanomechanics in health and disease: Lessons from titin

Figures

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  Fig. 1. Schematic depiction of the contracted (top) and stretched (bottom) sarcomere (not to scale). Titin is colored in ocher, while other sarcomeric proteins appear in grey. Immunoglobulin-like domains are represented as filled circles, and the approximate positions of the unstructured N2-Bus and PEVK domains are indicated. Domains unfolded under force during stretching appear as extended lines in the bottom graph. The whole extension of the mechanically active I-band and the beginning of the A-band are delimited by arrows.

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  Fig. 2. A: Representation of half a sarcomere (top, not to scale), indicating the positions of the I-band (red) and the A-band (beige) aligned with the cysteine content along the sequence of human titin (bottom) [15]. Immunoglobulin (Ig), unstructured and fibronectin III (Fn) domains are shown. B and C: 3-D homology models of immunoglobulin-like domains I15 (2620–2703) and I66 (8326–8414), respectively [15]. Cysteine side-chains are highlighted in red. Sequences correspond to uniprot entry Q8WZ42.

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  Fig. 3. Schematic AFM traces showing the mechanical effects of redox PTMs. A: When a single polyprotein made of repetitions of one of titin's Ig domains is stretched under force in the AFM (right), 26 nm stepwise increases in protein length are detected, corresponding to individual domain unfolding events (left, beige line). If a disulfide bond is formed between two cysteines, the stiffening of the domains is manifested by their limited extension, showing 10 nm steps (red line) [16]. B: When a polyprotein is extended in presence of oxidized glutathione (G-S-S-G), cryptic cysteines become exposed to the reagent and might be modified. Intact domains are able to refold when force is relaxed, and their unfolding is detected again in a second probe pulse (1), whereas modified domains are unable to refold leading to immediate full extension of the polyprotein at high force (2) [15].