Titin is a multifunctional filamentous protein anchored in the M-band, a hexagonally organized supramolecular lattice in the middle of the muscle sarcomere. Functionally, the M-band is a framework that cross-links myosin thick filaments, organizes associated proteins, and maintains sarcomeric symmetry via its structural and putative mechanical properties. Part of the M-band appears at the C-terminal end of isolated titin molecules in the form of a globular head, named here the “M-complex”, which also serves as the point of head-to-head attachment of titin. We used high-resolution atomic force microscopy and nanosurgical manipulation to investigate the topographical and internal structure and local mechanical properties of the M-complex and its associated titin molecules. We find that the M-complex is a stable structure that corresponds to the transverse unit of the M-band organized around the myosin thick filament. M-complexes may be interlinked into an M-complex array that reflects the local structural and mechanical status of the transversal M-band lattice. Local segments of titin and the M-complex could be nanosurgically manipulated to achieve extension and domain unfolding. Long threads could be pulled out of the M-complex, suggesting that it is a compact supramolecular reservoir of extensible filaments. Nanosurgery evoked an unexpected volume increment in the M-complex, which may be related to its function as a mechanical spacer. The M-complex thus displays both elastic and plastic properties which support the idea that the M-band may be involved in mechanical functions within the muscle sarcomere.

Von Willebrand factor (VWF) is a multimer with a variable number of protomers, each of which is a head-to-head dimer of two multi-domain monomers. VWF responds to shear through the unfolding and extension of distinct domains, thereby mediating platelet adhesion and aggregation to the injured blood vessel wall. VWF's C_1-6 segment uncoils and then the A₂ domain unfolds and extends in a hierarchical and sequential manner. However, it is unclear whether there is any reservoir of further extensibility. Here, we explored the presence of cryptic extensibility in VWF by nanodissecting individual, pre-stretched multimers with atomic force microscopy (AFM). The AFM cantilever tip was pressed into the surface and moved in a direction perpendicular to the VWF axis. It was possible to pull out protein loops from VWF, which resulted in a mean contour length gain of 217 nm. In some cases, the loop became cleaved, and a gap was present along the contour. Frequently, small nodules appeared in the loops, indicating that parts of the nanodissected VWF segment remained folded. After analyzing the nodal structure, we conclude that the cryptic extensibility lies within the C_1-6 and A_1-3 regions. Cryptic extensibility may play a role in maintaining VWF’s functionality in extreme shear conditions.

Understanding the structure and virion-host interaction of SARS-CoV-2 is crucial for elucidating the fundamental mechanisms of its assembly, stability, and transmission. These insights not only improve antiviral strategies but also contribute to a broader understanding of the nanoscale biological systems. Most studies of coronavirus focus primarily on viral genetics and on the structure and receptor affinity of the spike protein but overlook the broader mechanical and structural properties of the virion as a whole. Several studies have already suggested structural variability among coronavirus virions, and our work aims to provide more evidence of this matter. Here, we studied chemically fixed SARS-CoV-2 variants, focusing on wild-type, alpha, and delta variants. We used atomic force microscopy to acquire high-resolution topographic information. To estimate physically plausible virion envelope shapes, we utilized the Helfrich vesicle model as part of the analysis pipeline. We estimated viral geometry and adhesional compliance through reduced volume and its relationship with other geometrical parameters. We revealed consistent differences in apparent virion geometry across the three variants with alpha and delta displaying smaller fitted envelopes and lower reduced-volume estimates than wild-type under identical capture, fixation, and imaging conditions. Geometry-derived contact metrics also differed systematically among variants, consistent with differences in apparent deformation and compliance in this assay. Together, these descriptors establish a comparative framework for assessing the variant-dependent virion geometry and apparent deformation behavior.

BRIEF SUMMARY: The topic of the paper on a global scope is viral evolution, on a local scope it focuses on the recent SARS-CoV-2 pandemic. The paper proposes multiple models which could explain the dynamics of the ascension of new variants and how th...