Water dynamics in the hydration shell of the peripheral membrane protein annexin B12 were studied using MD simulations and Overhauser DNP-enhanced NMR. We show that retardation of water motions near phospholipid bilayers is extended by the presence of a membrane-bound protein, up to around 10 Å above that protein. Near the membrane surface, electrostatic interactions with the lipid head groups strongly slow down water dynamics, whereas protein-induced water retardation is weaker and dominates only at distances beyond 10 Å from the membrane surface. The results can be understood from a simple model based on additive contributions from the membrane and the protein to the activation free energy barriers of water diffusion next to the biomolecular surfaces. Furthermore, analysis of the intermolecular vibrations of the water network reveals that retarded water motions near the membrane shift the vibrational modes to higher frequencies, which we used to identify an entropy gradient from the membrane surface towards the bulk water. Our results have implications for processes that take place at lipid membrane surfaces, including molecular recognition, binding, and protein-protein interactions.

Immune recognition of infected or malignantly transformed cells relies on antigenic peptides exposed at the cell surface by major histocompatibility complex class I (MHC I) molecules. Selection and loading of peptides onto MHC I is orchestrated by the peptide-loading complex (PLC), a multiprotein assembly whose structure has not yet been resolved. Tapasin, a central component of the PLC, stabilises MHC I and catalyses the exchange of low-affinity against high-affinity, immunodominant peptides. Up to now, the molecular basis of this peptide editing mechanism remained elusive. Here, using all-atom molecular dynamics (MD) simulations, we unravel the atomic details of how tapasin and antigen peptides act on the MHC I binding groove. Force distribution analysis reveals an intriguing molecular tug-of-war mechanism: only high-affinity peptides can exert sufficiently large forces to close the binding groove, thus overcoming the opposite forces exerted by tapasin to open it. Tapasin therefore accelerates the release of low-affinity peptides until a high-affinity antigen binds, promoting subsequent PLC break-down. Fluctuation and entropy analyses show how tapasin chaperones MHC I by stabilising it in a peptide-receptive conformation. Our results explain previous experiments and mark a key step towards a better understanding of adaptive immunity.

The peptide-loading complex plays a pivotal role in Ag processing and is thus central to the efficient immune recognition of virally and malignantly transformed cells. The underlying mechanism by which MHC class I (MHC I) molecules sample immunodominant peptide epitopes, however, remains poorly understood. In this article, we delineate the interaction between tapasin (Tsn) and MHC I molecules. We followed the process of peptide editing in real time after ultra-fast photoconversion to pseudoempty MHC I molecules. Tsn discriminates between MHC I loaded with optimal and MHC I bound to suboptimal cargo. This differential interaction is key to understanding the kinetics of epitope proofreading. To elucidate the underlying mechanism at the atomic level, we modeled the Tsn/MHC I complex using all-atom molecular dynamics simulations. We present a catalytic working cycle, in which Tsn binds to MHC I with suboptimal cargo and thereby adjusts the energy landscape in favor of MHC I complexes with immunodominant epitopes.

No abstract