Single-residue physicochemical characteristics kinetically partition membrane protein self-assembly and aggregation

Studies on the Structure and Properties of Membrane Phospholipase A1 Inclusion Bodies Formed at Low Growth Temperatures Using GFP Fusion Strategy

The effect of cultivation temperatures (37, 26, and 18 °C) on the conformational quality of Yersinia pseudotuberculosis phospholipase A₁ (PldA) in inclusion bodies (IBs) was studied using green fluorescent protein (GFP) as a folding reporter. GFP was fused to the C-terminus of PldA to form the PldA-GFP chimeric protein. It was found that the maximum level of fluorescence and expression of the chimeric protein is observed in cells grown at 18 °C, while at 37 °C no formation of fluorescently active forms of PldA-GFP occurs. The size, stability in denaturant solutions, and enzymatic and biological activity of PldA-GFP IBs expressed at 18 °C, as well as the secondary structure and arrangement of protein molecules inside the IBs, were studied. Solubilization of the chimeric protein from IBs in urea and SDS is accompanied by its denaturation. The obtained data show the structural heterogeneity of PldA-GFP IBs. It can be assumed that compactly packed, properly folded, proteolytic resistant, and structurally less organized, susceptible to proteolysis polypeptides can coexist in PldA-GFP IBs. The use of GFP as a fusion partner improves the conformational quality of PldA, but negatively affects its enzymatic activity. The PldA-GFP IBs are not toxic to eukaryotic cells and have the property to penetrate neuroblastoma cells. Data presented in the work show that the GFP-marker can be useful not only as target protein folding indicator, but also as a tool for studying the molecular organization of IBs, their morphology, and localization in E. coli, as well as for visualization of IBs interactions with eukaryotic cells.

Evaluation of Peptide/Protein Self-Assembly and Aggregation by Spectroscopic Methods

The self-assembly of proteins is an essential process for a variety of cellular functions including cell respiration, mobility and division. On the other hand, protein or peptide misfolding and aggregation is related to the development of Parkinson's disease and Alzheimer's disease, among other aggregopathies. As a consequence, significant research efforts are directed towards the understanding of this process. In this review, we are focused on the use of UV-Visible Absorption Spectroscopy, Fluorescence Spectroscopy and Circular Dichroism to evaluate the self-organization of proteins and peptides in solution. These spectroscopic techniques are commonly available in most chemistry and biochemistry research laboratories, and together they are a powerful approach for initial as well as routine evaluation of protein and peptide self-assembly and aggregation under different environmental stimulus. Furthermore, these spectroscopic techniques are even suitable for studying complex systems like those in the food industry or pharmaceutical formulations, providing an overall idea of the folding, self-assembly, and aggregation processes, which is challenging to obtain with high-resolution methods. Here, we compiled and discussed selected examples, together with our results and those that helped us better to understand the process of protein and peptide aggregation. We put particular emphasis on the basic description of the methods as well as on the experimental considerations needed to obtain meaningful information, to help those who are just getting into this exciting area of research. Moreover, this review is particularly useful to those out of the field who would like to improve reproducibility in their cellular and biomedical experiments, especially while working with peptide and protein systems as an external stimulus. Our final aim is to show the power of these low-resolution techniques to improve our understanding of the self-assembly of peptides and proteins and translate this fundamental knowledge in biomedical research or food applications.

Evolutionary selection of a 19-stranded mitochondrial β-barrel scaffold bears structural and functional significance

Transmembrane β-barrels of eukaryotic outer mitochondrial membranes (OMMs) are major channels of communication between the cytosol and mitochondria and are indispensable for cellular homeostasis. A structurally intriguing exception to all known transmembrane β-barrels is the unique odd-stranded, i.e. 19-stranded, structures found solely in the OMM. The molecular origins of this 19-stranded structure and its associated functional significance are unclear. In humans, the most abundant OMM transporter is the voltage-dependent anion channel. Here, using the human voltage-dependent anion channel as our template scaffold, we designed and engineered odd- and even-stranded structures of smaller (V2¹⁶, V2¹⁷, V2¹⁸) and larger (V2²⁰, V2²¹) barrel diameters. Determination of the structure, dynamics, and energetics of these engineered structures in bilayer membranes reveals that the 19-stranded barrel surprisingly holds modest to low stability in a lipid-dependent manner. However, we demonstrate that this structurally metastable protein possesses superior voltage-gated channel regulation, efficient mitochondrial targeting, and in vivo cell survival, with lipid-modulated stability, all of which supersede the occurrence of a metastable 19-stranded scaffold. We propose that the unique structural adaptation of these transmembrane transporters exclusively in mitochondria bears strong evolutionary basis and is functionally significant for homeostasis.