The HIV-1 Protease (PR) mediates its own release and promotes the proteolysis of the Gag-Pol polyprotein into mature structural and functional forms indis- pensable for viral maturation. Due to its central role in the virus life cycle, the HIV-1 PR has been a prominent target for clinical protease inhibitors for almost 25 years. However the short replication cycle of the virus together with the error-prone reverse transcriptase lead to the rapidly evolving selection of inhibitor-resistant mutations in HIV-1 PR. We use here high-pressure NMR spectroscopy to characterize the cooperativity of unfolding, in the absence and in the presence of a symmetric inhibitor, of the active mature PR, the inactive variant PR D25N and of the clinical multi-drug resistant PR20. We found that binding of the inhibitor drastically decreases the cooperativity of unfolding by trapping the closed flap conformation in a deep free-energy minimum. We show that in order to avoid this conformational trap, PR20 has evolved to exhibit a nearly ideal two-state unfolding transition. Our study highlights the malleability of the HIV-1 PR folding pathways and illustrates how selection for drug resistant mutations can lead to a major remodeling of the free-energy landscape. 1818-Plat Folding Mechanism of a Metastable Serpin at Atomic Resolution Fang Wang 1 , Haiping Ke 2 , Silvio a Beccara 3 , Anne Gershenson 2 , Pietro Faccioli 4 , Patrick Wintrode 1 . 1 Pharmaceutical Sciences, University of Maryland, Baltimore, MD, USA, 2 Biochemistry and Molecular Biology, University of Massachusetts, Amherst, MA, USA, 3 LISC, Fondazione Bruno Kessler, Trento, Italy, 4 Physics, Trento University, Trento, Italy. Inhibitory serpins are conformationally labile proteins that fold to a kineti- cally trapped, metastable state. Disease linked naturally occurring mutations can result in misfolding and polymerization and the folding and misfolding mechanisms of serpins are therefore of intense medical interest. However, at nearly 400 amino acids, serpins are much too large for their folding to be simulated using conventional molecular dynamics. Recently, the Domi- nant Reaction Pathways (DRP) method, a path integral based variational framework, has proven capable of simulating protein folding with extremely high computational efficiency. Utilizing this method we have simulated the folding of the serpin alpha-1 antitrypsin (A1AT) in all atom detail using a physics based force field. We find that folding begins with the independent formation of local ‘‘foldons’’, and these foldons then dock to each other in a defined order to successfully reach the native state. Introducing the most common disease associated mutation (the ‘‘Z’’ mutation E342K) leads to a state in which two of the three beta sheets, A and B, fail to fully form. In the simulations, a key event in Z misfolding is the early failure of beta strands 5 and 6A to dock to a region called the B-C barrel, indicating that the folding pathways of wildtype (WT) and Z A1AT diverge much earlier than was generally supposed. Our proposed WT folding pathway and structure for mis- folded Z are in excellent agreement with published experimental data. Intro- ducing Ala at position 290 in the Z mutant promotes folding in our simulations, and preliminary experimental data indicate that this mutation may suppress Z polymerization in cells. These results show that the DRP method can reveal the folding mechanisms of large complex proteins in un- precedented detail without resort to coarse graining or structure based force fields. 1819-Plat NMR-Informed Molecular Modeling Uncovers the Conformational Land- scape of Chaperone Binding with Unfolded Substrate Logan S. Ahlstrom 1 , Loı ¨c Salmon 2 , Scott Horowitz 2 , Alex Dickson 1 , Charles L. Brooks III, 3 , James C.A. Bardwell 2 . 1 Department of Chemistry, University of Michigan, Ann Arbor, MI, USA, 2 Department of Molecular, Cellular and Developmental Biology, and the Howard Hughes Medical Institute, University of Michigan, Ann Arbor, MI, USA, 3 Department of Chemistry and Biophysics Program, University of Michigan, Ann Arbor, MI, USA. Molecular chaperones interact with unfolded proteins to prevent aberrant folding and aggregation in the crowded cellular milieu. Despite their crit- ical role in maintaining protein homeostasis, achieving detailed models of the highly dynamic interaction between a chaperone and an unfolded sub- strate has proven to be a difficult task. We address this challenge by devising a methodology that fuses site-specific NMR information with coarse-grained molecular dynamics simulations to achieve a residue-level description of chaperone-substrate interaction. As a model system, we decided to consider the interaction of the small, ATP-independent chap- erone Spy with its in vivo substrate Im7. Specifically, we develop an auto- mated procedure that optimizes coarse-grained force fields for the individual partners against experimentally measured carbon chemical shifts. Additional NMR data is used to validate the models. We then combine the force fields of the two binding partners with a generic inter-protein potential to perform simulations of the dynamic chaperone-substrate complex. We observe an overall decrease in global dynamics of the Spy chaperone that is counter-balanced by an increase in the local dynamics in its flexible loop region. Increased loop flexibility appears to facilitate chaperone inter- action with multiple partially folded states that are sampled by the Im7 substrate. Moreover, interaction with the Spy chaperone slows down conformational exchange in the Im7 substrate, while also shifting the sub- strate folding landscape toward more structured states. Our hybrid strategy provides an avenue to investigate other heterogeneous biomolecular com- plexes through the integration of NMR data with efficient computational models. Platform: Membrane Physical Chemistry III 1820-Plat Elastic Deformation and Collective Dynamics in Lipid Membranes: A Solid-State 2 H NMR Relaxation Study Soohyun K. Lee 1 , Trivikram R. Molugu 1 , K.J. Mallikarjunaiah 1,2 , Michael F. Brown 1,3 . 1 Department of Chemistry and Biochemistry, University of Arizona, Tucson, AZ, USA, 2 Department of Physics, Indian Institute of Science, Bangalore, India, 3 Department of Physics, University of Arizona, Tucson, AZ, USA. Biomembrane functioning is significantly influenced by the composition and structure of the liquid-crystalline lipid bilayer [1]. Solid-state 2 H NMR spec- troscopy provides information about atomistic interactions among the mem- brane constituents by simultaneously probing structure and dynamics [2]. Here we examine the effect of water, osmolytes, cholesterol, and detergents on the liquid-crystalline properties of lipid membranes using NMR relaxa- tion methods. We performed 2 H NMR longitudinal (R 1Z ) and transverse quadrupolar-echo decay (R 2 QE ) experiments on DMPC-d 54 bilayers to study membrane lipid dynamics over the time scale ranging from nanoseconds to milliseconds. Plots of R 1Z rates or transverse relaxation rates versus squared segmental order parameters (S CD 2 ) show the emergence of collective lipid dynamics [3]. Such a functional behavior characterizes 3-D order-director fluctuations, due to the onset of membrane elasticity [3]. Yet at high hydra- tion, a further R 2 QE enhancement and confinement of the functional square- law to the segments deeper in the bilayer is seen. Additional contributions from slower dynamics involving water-mediated membrane deformation are evident over mesoscopic length scales on the order of bilayer thickness. Such structural deformations are also evident from bilayer structural param- eters calculated using a statistical mean-torque model [4]. The slow dy- namics at high hydration or correspondingly low cholesterol or osmolyte concentration are due to modulation of elastic properties of the lipid bilayer. Analysis of the frequency dispersion of the transverse relaxation as a function of such external parameters reveals viscoelastic properties of the liquid-crystalline membranes. Such studies give insights into lipid rafts and membrane composition relevant for biomembrane function. [1] A. Leftin et al. (2014) Biophys. J. 107, 2274[2] K.J. Mallikarjuniah et al. (2011) Biophys. J.100, 98. [3] A. Leftin et al. (2014) eMagRes 4, 199. [4] J.J. Kinnun et al. (2015) BBA 1848, 246. 1821-Plat Cholesterol Effect on the Elastic Properties of Unsaturated Lipid Bilayers Pavel Bashkirov 1,2 , Ksenia Chekashkina 2 , Ariana Velasco del Olmo 3 , Piotr Kuzmin 2 , Anna Shnyrova 3 , Vadim Frolov 3,4 . 1 Laboratory of Nanomedcine, Sceintific Research Institute of Physical- Chemical Medcine FMBA of Russia, Moscow, Russian Federation, 2 Frumkin Insititute of Physical Chemistry and Electrochemistry of RAS, Moscow, Russian Federation, 3 Biophysics Unit (CSIC, UPV/EHU) and Department of Biochemistry and Molecular Biology, University of the Basque Country, Leioa, Spain, 4 IKERBASQUE, Basque Foundation for Science, Bilbao, Spain. Cholesterol is one of the major determinants of the structure and function of cellular membranes. Interaction of cholesterol with lipid molecules affects diverse membrane properties, such as elasticity, fluidity, dynamics of mem- brane proteins and lipids. Despite decades of studies, the effects of choles- terol on lipid bilayer are not fully understood, especially in the context of liquid disordered bilayers omnipresent in cellular membranes. We demon- strate here that cholesterol can effectively interact with unsaturated lipid mol- ecules and, via these interactions, regulate bending stiffness of lipid bilayers made of unsaturated lipids of different molecular geometry. Using highly Tuesday, March 1, 2016 369a