ABSTRACTLiving matter is a quasi-stationary out-of-equilibrium system; in this physical condition, structural fluctuations at nano- and meso-scales are needed to understand the physics behind its biological functionality. Myelin has a simple ultrastructure whose fluctuations show correlated disorder in its functional out-of-equilibrium state. However, there is no information on the relationship between this correlated disorder and the dynamics of the intrinsically disordered Myelin Basic Protein (MBP) which is expected to influence the membrane structure and overall functionality. In this work, we have investigated the role of this protein structural dynamics in the myelin ultrastructure fluctuations in and out-of-equilibrium conditions, by using synchrotron Scanning micro X Ray Diffraction and Small Angle X ray Scattering. We have induced the crossover from out-of-equilibrium functional state to in-equilibrium degeneration changing the pH far away from physiological condition. While the observed compression of the cytosolic layer thickness probes the unfolding of the P2 protein and of the cytoplasmic P0 domain (P0cyt), the intrinsic large MBP fluctuations preserve the cytosol structure also in the degraded state. Thus, the transition of myelin ultrastructure from correlated to uncorrelated disordered state, is significantly affected by the unfolding of the P2 and P0 proteins, which in this latter state do not act in synergistic manner with MBP to determine the membrane functionality.STATEMENT OF SIGNIFICANCEA better comprehension of myelin degenerative process and the role of protein dynamics in this biological membrane is a topic issue in today’s scientific community. The myelin ultrastructural fluctuations exhibit correlated disorder in its functional state, that becomes uncorrelated as it degenerates. In this work we elucidate the interplay of protein structural dynamics and myelin ultrastructure in the transition from its functional state to the degraded state. The results highlight that the intrinsically disordered Myelin Basic Protein (MBP) allows to preserve the myelin structure following both the small correlated fluctuations in physiological state and the large disordered fluctuations in degraded conditions, where the myelin functionality is close to being lost and the MBP remains the single active protein.
Direct Observation of Cooperative Protein Structural Dynamics of Homodimeric Hemoglobin from 100 ps to 10 ms with Pump–Probe X-ray Solution Scattering