Interaction of DWORF with SERCA and PLB as determined by EPR spectroscopy

Insufficient sarco/endoplasmic reticulum calcium ATPase (SERCA) activity significantly contributes to heart failure, which is a leading cause of death worldwide. A characteristic pathology of cardiac disease is the slow and incomplete Ca2+ removal from the myocyte cytoplasm in diastole, which is primarily driven by SERCA, the integral transmembrane Ca2+ pump. Phospholamban (PLB) allosterically inhibits SERCA by reducing its apparent Ca2+ affinity. Recently, the 34-codon novel dwarf open reading frame (DWORF) micropeptide has been identified as a muscle-specific SERCA effector, capable of reversing the inhibitory effects of PLB and independently activating SERCA in the absence of PLB. However, the structural basis for these functions has not yet been determined in a system of defined molecular components. We have used electron paramagnetic resonance (EPR) spectroscopy to investigate the protein-protein interactions of DWORF, co-reconstituted in proteoliposomes with SERCA and spin-labeled PLB. We analyzed the change of PLB rotational mobility in response to varying DWORF concentration, to quantify competitive binding of DWORF and PLB. We determined that DWORF competes with PLB for binding to SERCA at low [Ca2+], although the measured affinity of DWORF for SERCA is an order of magnitude weaker than that of PLB for SERCA, indicating cooperativity. The sensitivity of EPR to structural dynamics, using stereospecifically attached spin labels, allows us to obtain new information needed to refine the molecular model for regulation of SERCA activity, as needed for development of novel therapeutic remedies against cardiac pathologies.

Tracking Membrane Protein Dynamics in Real Time

Abstract

Membrane proteins govern critical cellular processes and are central to human health and associated disease. Understanding of membrane protein function is obscured by the vast ranges of structural dynamics—both in the spatial and time regime—displayed in the protein and surrounding membrane. The membrane lipids have emerged as allosteric modulators of membrane protein function, which further adds to the complexity. In this review, we discuss several examples of membrane dependency. A particular focus is on how molecular dynamics (MD) simulation have aided to map membrane protein dynamics and how enhanced sampling methods can enable observing the otherwise inaccessible biological time scale. Also, time-resolved X-ray scattering in solution is highlighted as a powerful tool to track membrane protein dynamics, in particular when combined with MD simulation to identify transient intermediate states. Finally, we discuss future directions of how to further develop this promising approach to determine structural dynamics of both the protein and the surrounding lipids.

Graphic Abstract

Single Molecule Tracking and Localization of Mitochondrial Protein Complexes in Live Cells

Mitochondria are the power plant of most non-green eukaryotic cells. An understanding of their function and regulation is only possible with the knowledge of the spatiotemporal dynamics of their proteins. Mitochondrial membrane proteins involved in diverse functions like protein import, cell respiration, metabolite transport, and mitochondrial morphology are mobile within membranes. Here, we provide a protocol for a superresolution fluorescence microscopy technique named tracking and localization microscopy (TALM) that allows for localization and diffusion analysis of single mitochondrial membrane proteins in situ in cell cultures. This noninvasive imaging technique is a useful tool to reveal the spatiotemporal organization of proteins in diverse mitochondrial membrane compartments in living cells. Proteins of interest are tagged with the HaloTag^® and specifically labeled with functionalized rhodamine dyes. The method profits from low abundance of proteins and therefore works better with substoichiometric labeling of HaloTag®-tagged proteins. In particular, the use of photostable bright rhodamine dyes enables the specific tagging and localization of single molecules with a calculated precision below 20 nm and the recording of single trajectories.