Bending-torsional elasticity and energetics of the plus-end microtubule tip

Microtubules (MTs), mesoscopic cellular filaments, grow primarily by the addition of GTP-bound tubulin dimers at their dynamic flaring plus-end tips. They operate as chemomechanical energy transducers with stochastic transitions to an astounding shortening motion upon hydrolyzing GTP to GDP. Time-resolved dynamics of the MT tip - a key determinant of this behavior - as a function of nucleotide state, internal lattice strain, and stabilizing lateral interactions have not been fully understood. Here, we use atomistic simulations to study the spontaneous relaxation of complete GTP-MT and GDP-MT tip models from unfavorable straight to relaxed splayed conformations and to comprehensively characterize the elasticity of MT tips. Our simulations reveal the dominance of viscoelastic dynamics of MT protofilaments during the relaxation process, driven by the stored bending-torsional strain and counterbalanced by the inter-protofilament interactions. We show that the post-hydrolysis MT tip is exposed to higher activation energy barriers for straight lattice formation, which translates into its inability to elongate. Our study provides an 'information ratchet' mechanism for the elastic energy conversion and release by MT tips and offers new insights into the mechanoenzymatics of MTs.

Dynamics of lattice defects in mixed DNA origami monolayers

DNA origami lattice formation at solid–liquid interfaces is surprisingly resilient toward the incorporation of DNA origami impurities with different shapes.

Biochemical Reconstitution of HIV-1 Assembly and Maturation

ABSTRACT The assembly of an orthoretrovirus such as HIV-1 requires the coordinated functioning of multiple biochemical activities of the viral Gag protein. These activities include membrane targeting, lattice formation, packaging of the RNA genome, and recruitment of cellular cofactors that modulate assembly. In most previous studies, these Gag activities have been investigated individually, which provided somewhat limited insight into how they functionally integrate during the assembly process. Here, we report the development of a biochemical reconstitution system that allowed us to investigate how Gag lattice formation, RNA binding, and the assembly cofactor inositol hexakisphosphate (IP6) synergize to generate immature virus particles in vitro. The results identify an important rate-limiting step in assembly and reveal new insights into how RNA and IP6 promote immature Gag lattice formation. The immature virus-like particles can be converted into mature capsid-like particles by the simple addition of viral protease, suggesting that it is possible in principle to fully biochemically reconstitute the sequential processes of HIV-1 assembly and maturation from purified components. IMPORTANCE Assembly and maturation are essential steps in the replication of orthoretroviruses such as HIV-1 and are proven therapeutic targets. These processes require the coordinated functioning of the viral Gag protein’s multiple biochemical activities. We describe here the development of an experimental system that allows an integrative analysis of how Gag’s multiple functionalities cooperate to generate a retrovirus particle. Our current studies help to illuminate how Gag synergizes the formation of the virus compartment with RNA binding and how these activities are modulated by the small molecule IP6. Further development and use of this system should lead to a more comprehensive understanding of the molecular mechanisms of HIV-1 assembly and maturation and may provide new insights for the development of antiretroviral drugs.