Exploring the mechanical stability of the C2 domains in human synaptotagmin 1

Human synaptotagmin 1 (Syt1) plays a crucial role in the bending of the membrane during neurotransmitter release at the synapse. Hence, resolving the structural details of Syt1 that underlie its biological function is fundamental for providing mechanistic insights into the nature of the synaptic response. We explored the unfolding micromechanics of Syt1 by analyzing the free energy landscape of the whole molecule and its C2A and C2B domains. We employed a self-organized polymer (SOP) model of a protein chain to carry out pulling simulations, accelerated on graphics processing units (GPUs), under experimental force loads. To resolve the atomic-level details, we complemented the SOP model simulations with atomistic simulations. On the basis of the results obtained, we hypothesize that (1) isolated single domains C2A and C2B present similar mechanical resistance against an applied pulling force but unfold following different kinetic pathways and that (2) C2B is more mechanically resistant in the C2AB complex due to stabilizing interactions with other domains. These findings correlate well with recent atomic force microscopy (AFM) studies on the Syt1 molecule, in which the increase in the unfolding force for C2B was detected when this domain was joined with C2A. Our results also suggest that the linkers (I27 domains) used in the experimental setup can modulate the mechanical behavior of this synaptic protein complex and alter not only the critical force for unfolding but also the unfolding pathways for the C2 domains. Interestingly, the presence of the C2A-C2B domain interface in the C2AB complex confers mechanical stability to either of the C2 domains. Our findings provide new insights into the relative conformational variability of the C2 domains, which we believe to be modulated, to a large extent, by intermolecular coupling with other proteins.