title: A highly compliant protein native state with a spontaneous-like mechanical unfolding pathway.
abstract: The mechanical properties of proteins and their force-induced structural changes play key roles in many biological processes. Previous studies have shown that natively folded proteins are brittle under tension, unfolding after small mechanical deformations, while partially folded intermediate states, such as molten globules, are compliant and can deform elastically a great amount before crossing the transition state barrier. Moreover, under tension proteins appear to unfold through a different sequence of events than during spontaneous unfolding. Here, we describe the response to force of the four-α-helix acyl-CoA binding protein (ACBP) in the low-force regime using optical tweezers and ratcheted molecular dynamics simulations. The results of our studies reveal an unprecedented mechanical behavior of a natively folded protein. ACBP displays an atypical compliance along two nearly orthogonal pulling axes, with transition states located almost halfway between the unfolded and folded states. Surprisingly, the deformability of ACBP is greater than that observed for the highly pliant molten globule intermediate states. Furthermore, when manipulated from the N- and C-termini, ACBP unfolds by populating a transition state that resembles that observed during chemical denaturation, both for structure and position along the reaction coordinate. Our data provide the first experimental evidence of a spontaneous-like mechanical unfolding pathway of a protein. The mechanical behavior of ACBP is discussed in terms of topology and helix propensity.
authors.affiliation: Structural Biology and NMR Laboratory, Department of Biology, University of Copenhagen, Ole Maaløes Vej 5, 2200 Copenhagen N, Denmark.
authors.forename: Pétur O
authors.initials: PO
authors.lastname: Heidarsson
authors.forename: Immanuel
authors.initials: I
authors.lastname: Valpapuram
authors.forename: Carlo
authors.initials: C
authors.lastname: Camilloni
authors.forename: Alberto
authors.initials: A
authors.lastname: Imparato
authors.forename: Guido
authors.initials: G
authors.lastname: Tiana
authors.forename: Flemming M
authors.initials: FM
authors.lastname: Poulsen
authors.forename: Birthe B
authors.initials: BB
authors.lastname: Kragelund
authors.forename: Ciro
authors.initials: C
authors.lastname: Cecconi
mesh_terms: D026261:Diazepam Binding Inhibitor / Q000737:chemistry* / Q000302:isolation & purification; D008958:Models, Molecular; D056004:Molecular Dynamics Simulation; D058767:Protein Unfolding
chemical_list: D026261:Diazepam Binding Inhibitor