Fig 1.
Similar domain organization of crystallized collagen-binding MSCRAMM adhesins.
The collagen-binding adhesin of Staphylococcus aureus, CNA, has similar domain structure as many other bacterial adhesins. Therefore, CNA serves as a model protein that will share functional binding characteristics as other bacteria. The following are abbreviated: S—signal peptide; N1, N2, N3—collagen-binding region; B—repeats region; W—cell wall-anchoring region; M—trans-membrane region; and C—cytoplasmic tail. As evident, CNA is a characteristic adhesin in which similar proteins can be found across many species of bacteria.
Fig 2.
Binding mechanism of bacterial adhesin, CNA, with extracellular matrix protein, collagen.
(A) Hypothetical “Collagen Hug” schematic [17] demonstrating the (i) open state with initial collagen association with the N2 domain (ii) wrapping of collagen by the linker and N1 domain (iii) final closed state with the N1 domain interacting with the N2 domain and also the latch region secured in a groove in the N1 domain. (B) The CNA-collagen complex. The N1 and N2 globular domains along with their linker/hinge are shown wrapped around collagen. (C) Particular residues in CNA involved in interaction with collagen are highlighted and labeled. The collagen peptides are segmented by residue type with basic, non-polar, polar, and hydroxyproline residues colored as silver, yellow, green, and blue respectively.
Fig 3.
CNA N2 latch dissociation from ridge in N1 domain requires larger energy than provided by thermal energy.
(A) Electrostatic, van der Waals, and total interaction energy is shown for the Latch—N1 domain interaction. (B) Potential mean force (PMF) profile of latch dissociation from N1 domain. N2 latch dissociation from N1 domain was endothermic, requiring nearly 28 kBT of free energy—indicating that latch dissociation may require active processes and not solely through surrounding thermal energy.
Fig 4.
Inducing an in silico open-state CNA and demonstrating the natural frequencies of the CNA N1-N2 domains.
(A) The open conformation of CNA (right) was obtained by performing Steered Molecular Dynamics on the closed CNA conformation (left). (B) After equilibrating the open-state CNA, normal mode analysis on the CNA domains revealed modes with bending, twisting, and extending motions. Shown are the three lowest vibrational frequency modes that are the distinct modes of movement indicated by the arrows. The linker region between the N1 and N2 domains serves as a highly flexible hinge.
Fig 5.
Mechanoregulation of bacterial binding to collagen via applied tensile load.
(A) SMD simulation illustrating the tensile force applied to collagen while in complex with the N1-N2 domains of CNA. (B) Plot of an individual SMD simulation trial, in this case with 400 pN load, for interaction energy between the CNA N1-N2 domains and collagen. The data is sampled once the interaction energy stabilizes—region denoted by the shaded box. These windows in aggregate are displayed in part C. (C) Distribution of interaction energies for the CNA-collagen complex in equilibrium and under 80 pN, 400 pN, 600 pN, and 800 pN of tensile force applied to collagen. Results suggest that susceptibility of bacterial adhesion to collagen decreases under higher tensile forces in the body.
Fig 6.
Verification of the biological relevance of applied tensile forces.
(A) The average strain vs. load curve demonstrating the force dependence on length elongation of collagen. (B) Calculated average bond energies for the individual collagen peptides.
Fig 7.
Reversibility of collagen binding post-release of 800 pN force on collagen.
The red line plots the average interaction energy value for multiple trials and the dotted lines demonstrate maximum and minimum boundaries. The graph implies that the higher end of forces in the study is physiologically feasible.