Force-dependent Unfolding And Folding Dynamics Of ?-catenin Modulation Domains

As a key protein in the cell-cell adherens junction,a-catenin works not only as a crosslinker to connect cadherins/beta-catenin with F-actin cytoskeleton,but also as a force sensor.The force-induced conformation change and unfolding of its modulation domains will absorb work done by internal and external force as a force buffer,and also expose of binding site of another important cell adhesion proteins called vinculin.Modulation domains M1-3 of ?-catenin have native conformation composed of ?-helix bundles.Each of these three modulations domains has four ?-helix.Usually the topological conformation,especially the force-bearing pulling geometry,plays an important role in mechanical stability of proteins.Modulation domains of ?-catenin unfold at forces less than 20 pN.Comparing with immunoglobulin domain 127 of titin which is composed of seven beta-strands,?-catenin is much less mechanical stable.From both previous equilibrium measurement of M1 domain and our measurement of the loading rate-dependent unfolding forces of M2-3 domains,the unfolding distance,which is defined as the extension difference between unfolding transition state and native state,is around 5-8 nm.It indicates that the extension of unfolding transition vstate is almost twice of native state.Long unfolding distance xu of a-catenin makes its unfolding ery sensitive to stretching force.As a force sensor,the sensitivity is critical to its biological function.We estimate the parameter xf,the distance between unfolded peptide and the folding transition state at forces around 3.7pN is 13 nm,by optimizing the combined probability of all time traces.During the force decrease step,when all three domains are unfolded,equilibrium folding and unfolding dynamic process of M1 domain at 4-5 pN is not detected,which indicates the stability and folding dynamics of M1 domain depends on the conformation of M2-3 domains.During force-decreasing process,the first folding step has step size of more than 30 nm,which indicates that the folding of M1-3 domains is a cooperative process.