Biology subjects

Aramini, J. M.

Publications and source records attributed to Aramini, J. M..

5 recordsLinked to original sources

bioRxiv2026

The dynamic and heterogeneous structure of the non-canonical inflammasome

Inflammasomes are high molecular weight complexes that play an integral role in the innate immune system, triggering an inflammatory cascade to protect against cellular stresses such as pathogenic bacteria. Both canonical and non-canonical inflammasomes have been described in the literature and detailed structural studies of many components of the more complex and larger canonical versions have been reported. In contrast, corresponding structures of the non-canonical inflammasome have not emerged even though it consists of only two components: lipopolysaccharide (LPS) from gram-negative bacteria, and one of caspase-4 or caspase-5 in humans or caspase-11 in mice. Here we determine the stoichiometry of the non-canonical inflammasome using size-exclusion chromatography coupled with UV, refractive index, and light-scattering measurements, showing that the non-canonical inflammasome is heterogeneous, comprised of three major complexes with different numbers of LPS and caspase molecules. Solution Nuclear Magnetic Resonance (NMR) spectroscopy studies of the N-terminal Caspase Activation and Recruitment Domain (CARD) of caspase-11, that binds LPS, show that it is largely unstructured in the absence of lipid, with pervasive dynamics on the s-ms timescale. Formation of this complex increases the alpha-helical content of the CARD but the dynamics persist, multiple conformers are formed, and tertiary contacts are transient, consistent with formation of a molten globule. Our NMR results establish that the protease domain of caspase-11 is monomeric in isolation. As proteolysis is linked with dimerization, the protease domains are inactive in this state, but upon formation of the non-canonical inflammasome dimerization occurs, priming the complex for rapid processing of substrates. Significance StatementAnimals respond to injury, infection, or toxic materials via a process called inflammation. At the molecular level inflammation involves formation of large machines - inflammasomes - that are instrumental in triggering cascades that lead to an immune response. Although structural studies of canonical inflammasomes have emerged, much less is known about the structures of non-canonical inflammasomes. Using solution Nuclear Magnetic Resonance (NMR) spectroscopy in concert with other biophysical approaches we show that non-canonical inflammasomes are highly dynamic and structurally heterogeneous, and we characterize the different sized inflammasome particles that are formed in terms of their composition. We also show that once formed, non-canonical inflammasomes are primed to rapidly cleave substrate molecules, necessary for propagating the immune response.

biophysics↗

bioRxiv2020

Use of High Pressure NMR Spectroscopy to Rapidly Identify Proteins with Internal Ligand-Binding Voids

Small molecule binding within internal cavities provides a way to control protein function and structure, as exhibited in numerous natural and artificial settings. Unfortunately, most ways to identify suitable cavities require high-resolution structures a priori and may miss potential sites. Here we address this limitation via high-pressure solution NMR spectroscopy, taking advantage of the distinctive nonlinear pressure-induced chemical shift changes observed in proteins containing internal cavities and voids. We developed a method to rapidly characterize such nonlinearity among backbone 1H and 15N amide signals without needing to have sequence-specific chemical shift assignments, taking advantage of routinely available 15N-labeled samples, instrumentation, and 2D 1H/15N HSQC experiments. From such data, we find a strong correlation in the site-to-site variability in such nonlinearity with the total void volume within proteins, providing insights useful for prioritizing domains for ligand binding and indicating mode-of-action among such protein/ligand systems. We suggest that this experimental approach is a rapid and useful probe of otherwise hidden dynamic architectures of proteins, providing novel insights and opportunities into ligand binding and control. Significance StatementMany proteins can be regulated by internally binding small molecule ligands, but it is often not clear a priori which proteins are controllable in such a way. Here we describe a rapid method to address this challenge, using solution NMR spectroscopy to monitor the response of proteins to the application of high pressure. While the locations of NMR signals from many proteins respond to high pressure with linear chemical shift changes, proteins containing internal cavities that can bind small molecule ligands respond with easily identified non-linear changes. We demonstrate this approach on several proteins and protein/ligand complexes, suggesting that it has general utility.

biophysics↗

bioRxiv2020

High-Pressure NMR Reveals Volume and Compressibility Differences Between Two Stably Folded Protein Conformations

Proteins often interconvert between different conformations in ways critical to their function. While manipulating such equilibria for biophysical study is often challenging, the application of pressure is a potential route to achieve such control by favoring the population of lower volume states. Here, we use this feature to study the interconversion of ARNT PAS-B Y456T, which undergoes a dramatic beta-strand slip as it switches between two stably-folded conformations. Coupling high pressure and biomolecular NMR, we obtained the first quantitative data testing two key hypotheses of this process: the slipped conformation is both smaller and less compressible than the wildtype equivalent, and the interconversion proceeds through a chiefly-unfolded intermediate state. Our work exemplifies how these approaches, which can be generally applied to protein conformational switches, can provide unique information that is not easily accessible through other techniques.

biophysics↗

bioRxiv2020

Dynamics in Natural and Designed Elastins and Their Relation to Elastic Fiber Structure and Recoil

Elastin fibers assemble in the extracellular matrix from the precursor protein tropoelastin and provide the flexibility and spontaneous recoil required for arterial function. Unlike many proteins, a structure-function mechanism for elastin has been elusive. We have performed detailed NMR relaxation studies of the dynamics of the minielastins 24x' and 20x' using solution NMR, and of purified bovine elastin fibers in the presence and absence of mechanical stress using solid state NMR. The low sequence complexity of the minielastins enables us to determine dynamical timescales and degrees of local ordering with residue-specific resolution in the cross-link and hydrophobic modules using NMR relaxation. We find an extremely high degree of disorder, with order parameters for the entirety of the hydrophobic domains near zero, resembling that of simple chemical polymers and less than the order parameters that have been observed in other intrinsically disordered proteins. We find that backbone order parameters in natural, purified elastin fibers are comparable to those found in 24x' and 20x' in solution. The difference in dynamics, compared to the minielastins, is that backbone correlation times are significantly slowed in purified elastin. Moreover, when elastin is mechanically stretched, the high chain disorder in purified elastin is retained - showing that any change in local ordering is below that detectable in our experiment. Combined with our previous finding of a 10-fold increase in the ordering of water when fully hydrated elastin fibers are stretched by 50%, these results support the hypothesis that stretch induced solvent ordering, i.e., the hydrophobic effect, is a key player in the elastic recoil of elastin as opposed to configurational entropy loss. SIGNIFICANCEElastin is responsible for the spontaneous recoil of arterial walls that is necessary for cardiovascular function. Despite this critical role, the mechanism driving entropic recoil has remained unclear. Elastin is unusual in that it is intrinsically disordered in both soluble and fibrous forms. Using NMR, we have determined the timescales and amplitudes of dynamics in two soluble elastin mimetics and in relaxed and stretched states of purified bovine elastin fibers. Although dynamical timescales are different, both the soluble elastin mimetic and fibrillar elastin display an exceptionally high degree of disorder. No detectable increase in protein ordering was observed upon stretching, suggesting that entropic recoil is primarily driven by the hydrophobic effect and not configurational entropy loss.

biophysics↗

bioRxiv2020

Unraveling the Mechanism of a LOV Domain Optogenetic Sensor: A Glutamine Lever Induces Unfolding of the Jα Helix

Light-activated protein domains provide a convenient, modular, and genetically encodable sensor for optogenetics and optobiology. Although these domains have now been deployed in numerous systems, the precise mechanism of photoactivation and the accompanying structural dynamics that modulate output domain activity remain to be fully elucidated. In the C-terminal light, oxygen, voltage (LOV) domain of plant phototropins (LOV2), blue light activation leads to formation of an adduct between a conserved Cys residue and the embedded FMN chromophore, rotation of a conserved Gln (Q513), and unfolding of a helix (J-helix) which is coupled to the output partner. In the present work, we focus on the allosteric pathways leading to J helix unfolding in Avena sativa LOV2 (AsLOV2) using an interdisciplinary approach involving molecular dynamics simulations extending to 7 s, time-resolved infrared spectroscopy, solution NMR spectroscopy, and in-cell optogenetic experiments. In the dark state, the side chain of N414 is hydrogen bonded to the backbone N-H of Q513. The simulations predict a lever-like motion of Q513 after Cys adduct formation resulting in loss of the interaction between the side chain of N414 and the backbone C=O of Q513, and formation of a transient hydrogen bond between the Q513 and N414 side chains. The central role of N414 in signal transduction was evaluated by site-directed mutagenesis supporting a direct link between J helix unfolding dynamics and the cellular function of the Zdk2-AsLOV2 optogenetic construct. Through this multifaceted approach, we show that Q513 and N414 are critical mediators of protein structural dynamics, linking the ultrafast (sub-ps) excitation of the FMN chromophore to the microsecond conformational changes that result in photoreceptor activation and biological function.

biophysics↗