This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Titin (www.ks.uiuc.edu/Research/telethonin/) is a protein that protects muscle from overstretching, which can occur following a powerful muscle contraction. So important is this protective mechanism that defects in the titin gene have been correlated to the disease muscular distrophy, which is characterized by severe physical weakness. Titin functions as a molecular bungee cord, producing a restoring force when a muscle fiber is extended beyond its normal length. This prevents injury to muscle as a result of overextension. To perform its function, one end of titin called Z1Z2 must be attached to a rigid structure from which to stretch against. Very recently the atomic structure of the titin Z1Z2 bound to a protein called telethonin became available [164]. Unlike typical ligand-binding in which the ligand is inserted into a receptor pocket, the crystal structure for the titin Z1Z2-telethonin complex revealed that the receptor Z1Z2 domains of two separate antiparallel titin N-termini are joined together by an N-terminal fragment of the telethonin ligand through beta-strand cross-links, a structural motif that also appears in pathologies like protein fibrillation and plaque formation in Alzheimer s and Parkinson s disease [165 168]. Moreover, it is also known that beta-strand cross-links can form mechanically stable beta-sheets, for example in titin Ig domains and fibronectin type III-like modules. Naturally this raises the question: Does the beta-strand cross-linking between titin and telethonin represent a novel ligand-binding strategy that provides a mechanically stable linkage, and consequently does telethonin function as a rigid anchor for titin? This question has been addressed by all-atom Steered Molecular Dynamics simulations performed by the Resource using NAMD [44] in order to understand the mechanical design of the Z1Z2-telethonin complex [169]. In these simulations, the complex was stretched with an applied force from multiple directions simultaneously (a specific scheme adapted for this study) in order to measure the strength of the connections between titin and telethonin. The system size involved 140,000 atoms and revealed that the Z1 and Z2 domains are bound strongly to telethonin. Analysis using VMD [49] revealed that the beta-strand cross-links between titin Z1Z2 and telethonin can be thought of as a biological glue at the molecular level. It turns out, in fact, that the Z1Z2-telethonin complex exhibits significantly higher resistance to mechanical stretching forces than titin Z1Z2 alone, suggesting that telethonin plays a role in anchoring one end of titin. Whereas previous studies have revealed that the unraveling of the beta-strands for titin Ig-domains constitutes the dominant unfolding barrier [170], in the case of Z1Z2 in complex with telethonin, the force peak observed corresponded to a detachment of one virtually intact Z2 domain from a beta-strands of telethonin. The results from these simulations explain how the titin Z1Z2-telethonin complex produces extraordinary resistance to mechanical stress. The major force bearing component of this complex is an extensive intermolecular hydrogen bonding network formed across beta-strands between telethonin and Z1Z2 domains, and not intramolecularly between termini beta-strands of individual Z1 or Z2 domains. This shift to a stronger force bearing interface reduces the chance of unraveling the individual Ig-domains, thus stabilizing the complex. We found that telethonin functions as a strong molecular glue for augmenting the mechanical resilience of muscle proteins in order so that they can carry out their biological functions. A similar mechanism likely exists for the aggregation of pathological fibrils mentioned above.
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