Ca(2+) ATPase Conformational Transitions in Lipid Bilayers Mapped by Site-directed Ethylation and Solid-State NMR

Structural basis for allosteric control of the SERCA-Phospholamban membrane complex by Ca2+ and phosphorylation

Phospholamban (PLN) is a mini-membrane protein that directly controls the cardiac Ca²⁺-transport response to β-adrenergic stimulation, thus modulating cardiac output during the fight-or-flight response. In the sarcoplasmic reticulum membrane, PLN binds to the sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA), keeping this enzyme's function within a narrow physiological window. PLN phosphorylation by cAMP-dependent protein kinase A or increase in Ca²⁺ concentration reverses the inhibitory effects through an unknown mechanism. Using oriented-sample solid-state NMR spectroscopy and replica-averaged NMR-restrained structural refinement, we reveal that phosphorylation of PLN's cytoplasmic regulatory domain signals the disruption of several inhibitory contacts at the transmembrane binding interface of the SERCA-PLN complex that are propagated to the enzyme's active site, augmenting Ca²⁺ transport. Our findings address long-standing questions about SERCA regulation, epitomizing a signal transduction mechanism operated by posttranslationally modified bitopic membrane proteins.

Cysteine-ethylation of tissue-extracted membrane proteins as a tool to detect conformational states by solid-state NMR spectroscopy

Solid-state NMR (ssNMR) is an ideal tool to study structure and dynamics of membrane proteins in their native lipid environment. In principle, ssNMR has no size limitations. However, this feature is rarely exploited as large membrane proteins display severe resonance overlap. In addition, dismal yields from recombinant bacterial expression systems limit severely spectroscopic characterization of membrane proteins. For very large mammalian membrane proteins, extraction from the original organism remains the most viable approach. In this case, NMR-observable nuclei must be introduced post-translationally, but the approaches developed so far are rather scarce. Here, we detail the synthesis and engineering of a reactive ¹³C-ethylmethanethiosulfonate (¹³C-EMTS) reagent for the post-translational alkylation of cysteine sidechains of a 110kDa sarcoplasmic reticulum Ca²⁺-ATPase (SERCA) extracted from rabbit skeletal muscle tissue. When reconstituted into liposomes, it is possible to resolve the resonances of the engineered ethyl groups by magic-angle spinning (MAS) 2D [¹³C,¹³C]-DARR experiments. Notably, the ethyl-group modification does not perturb the function of SERCA, yielding well-resolved ¹³C-¹³C fingerprints that are used to image its structural states in the catalytic cycle and filtering out overwhelming naturally-abundant ¹³C nuclei signals arising from the enzyme and lipids. We anticipate that this approach will be used together with ¹⁹F NMR to monitor conformational transitions of enzymes and proteins that are difficult to express recombinantly.

Experimental Aspects of Polarization Optimized Experiments (POE) for Magic Angle Spinning Solid-State NMR of Microcrystalline and Membrane-Bound Proteins

Conventional NMR pulse sequences record one spectrum per experiment, while spending most of the time waiting for the spin system to return to the equilibrium. As a result, a full set of multidimensional NMR experiments for biological macromolecules may take up to several months to complete. Here, we present a practical guide for setting up a new class of MAS solid-state NMR experiments (POE or polarization optimized experiments) that enable the simultaneous acquisition of multiple spectra of proteins, accelerating data acquisition. POE exploit the long-lived ¹⁵N polarization of isotopically labeled proteins and enable one to obtain up to eight spectra, by concatenating classical NMR pulse sequences. This new strategy propels data throughput of solid-state NMR spectroscopy of fibers, microcrystalline preparations, as well as membrane proteins.

NMR as a tool to investigate the structure, dynamics and function of membrane proteins

Membrane-protein NMR occupies a unique niche for determining structures, assessing dynamics, examining folding, and studying the binding of lipids, ligands and drugs to membrane proteins. However, NMR analyses of membrane proteins also face special challenges that are not encountered with soluble proteins, including sample preparation, size limitation, spectral crowding and sparse data accumulation. This Perspective provides a snapshot of current achievements, future opportunities and possible limitations in this rapidly developing field.

Orphan spin polarization: A catalyst for high-throughput solid-state NMR spectroscopy of proteins

Magic angle spinning solid-state NMR (MAS ssNMR) spectroscopy is a powerful method for structure determination of biomacromolecules that are recalcitrant to crystallization (membrane proteins and fibrils). Conventional multidimensional ssNMR methods acquire one experiment at a time. This approach is time consuming and discards orphan (unused) spin operators. Relatively low sensitivity and poor resolution of protein samples require long acquisition times for multidimensional ssNMR experiments. Here, we describe our recent progress in the development of multiple acquisition solid-state NMR methods for protein structure determination. A family of experiments called Polarization Optimized Experiments (POE) were designed, in which we utilized the orphan spin operators that are discarded in classical multidimensional NMR experiments, recovering them to allow simultaneous acquisition of multiple 2D and 3D experiments, all while using conventional probes with spectrometers equipped with one receiver. Three strategies namely, DUMAS, MEIOSIS, and MAeSTOSO were used for the concatenation of various 2D and 3D experiments. These methods open up new avenues for reducing the acquisition times of multidimensional experiments for biomolecular ssNMR spectroscopy.