An investigation into the protonation states of the C1 domain of cardiac myosin-binding protein C

Myosin-binding protein C (MyBP-C) is a myofibril-associated protein found in cardiac and skeletal muscle. The cardiac isoform (cMyBP-C) is subject to reversible phosphorylation and the surface-charge state of the protein is of keen interest with regard to understanding the inter-protein interactions that are implicated in its function. Diffraction data from the C1 domain of cMyBP-C were extended to 1.30 Å resolution, where the 〈I/σ(I)〉 of the diffraction data crosses 2.0, using intense synchrotron radiation. The protonation-state determinations were not above 2σ (the best was 1.81σ) and therefore an extrapolation is given, based on 100% data completeness and the average DPI, that a 3σ determination could be possible if X-ray data could be measured to 1.02 Å resolution. This might be possible via improved crystallization or multiple sample evaluation, e.g. using robotics or a yet more intense/collimated X-ray beam or combinations thereof. An alternative would be neutron protein crystallography at 2 Å resolution, where it is estimated that for the unit-cell volume of the cMyBP-C C1 domain crystal a crystal volume of 0.10 mm³ would be needed with fully deuterated protein on LADI III. These efforts would optimally be combined in a joint X-ray and neutron model refinement.