Interfacial Properties of NTAIL, an Intrinsically Disordered Protein

Location of the photosynthetic carbon metabolism in microcompartments and separated phases in microalgal cells

Carbon acquisition, assimilation and storage in eukaryotic microalgae and cyanobacteria occur in multiple compartments that have been characterised by the location of the enzymes involved in these functions. These compartments can be delimited by bilayer membranes, such as the chloroplast, the lumen, the peroxisome, the mitochondria or monolayer membranes, such as lipid droplets or plastoglobules. They can also originate from liquid-liquid phase separation such as the pyrenoid. Multiple exchanges exist between the intracellular microcompartments, and these are reviewed for the CO2 concentration mechanism, the Calvin-Benson-Bassham cycle, the lipid metabolism and the cellular energetic balance. Progress in microscopy and spectroscopic methods opens new perspectives to characterise the molecular consequences of the location of the proteins involved, including intrinsically disordered proteins.

Aβ-Amyloid Fibrils Are Self-Triggered by the Interfacial Lipid Environment and Low Peptide Content

We studied the surface properties of Aβ(1-40) amyloid peptides mixed with 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) (liquid state) or 1,2-disteraoyl-phosphatidylcholine (DSPC) (solid state) phospholipids by using nanostructured lipid/peptide films (Langmuir monolayers). Pure Aβ(1-40) amyloid peptides form insoluble monolayers without forming fibril-like structures. In a lipid environment [phospholipid/Aβ(1-40) peptide mixtures], we observed that both miscibility and stability of the films depend on the peptide content. At low Aβ(1-40) amyloid peptide proportion (from 2.5 to 10% of peptide area proportion), we observed the formation of a fibril-like structure when mixed only with POPC lipids. The stability acquired by these mixed films is within 20-35 mN·m^-1 compatible with the equivalent surface pressure postulated for natural biomembranes. Fibrils are clearly evidenced directly from the monolayers by using Brewster angle microscopy. The so-called nanostructured fibrils are thioflavin T positive when observed by fluorescence microscopy. The amyloid fibril network at the surface was also evidenced by atomic force microscopy when the films are transferred onto a mica support. Aβ(1-40) amyloid mixed with the solid DSPC lipid showed an immiscible behavior in all peptide proportions without fibril formation. We postulated that the amyloid fibrillogenesis at the membrane can be dynamically nano-self-triggered at the surface by the quality of the interfacial environment, that is, the physical state of the water-lipid interface and the relative content of amyloid protein present at the interface.

Orchestration of algal metabolism by protein disorder

Intrinsically disordered proteins (IDPs) are proteins that provide many functional advantages in a large number of metabolic and signalling pathways. Because of their high flexibility that endows them with pressure-, heat- and acid-resistance, IDPs are valuable metabolic regulators that help algae to cope with extreme conditions of pH, temperature, pressure and light. They have, however, been overlooked in these organisms. In this review, we present some well-known algal IDPs, including the conditionally disordered CP12, a protein involved in the regulation of CO₂ assimilation, as probably the best known example, whose disorder content is strongly dependent on the redox conditions, and the essential pyrenoid component 1 that serves as a scaffold for ribulose-1, 5-bisphosphate carboxylase/oxygenase. We also describe how some enzymes are regulated by protein regions, called intrinsically disordered regions (IDRs), such as ribulose-1, 5-bisphosphate carboxylase/oxygenase activase, the A₂B₂ form of glyceraldehyde-3-phosphate dehydrogenase and the adenylate kinase. Several molecular chaperones, which are crucial for cell proteostasis, also display significant disorder propensities such as the algal heat shock proteins HSP33, HSP70 and HSP90. This review confirms the wide distribution of IDPs in algae but highlights that further studies are needed to uncover their full role in orchestrating algal metabolism.

Order in Disorder as Observed by the "Hydrophobic Cluster Analysis" of Protein Sequences

Hydrophobic cluster analysis (HCA) is an original approach for protein sequence analysis, which provides access to the foldable repertoire of the protein universe, including yet unannotated protein segments ("dark proteome"). Foldable segments correspond to ordered regions, as well as to intrinsically disordered regions (IDRs) undergoing disorder to order transitions. In this review, how HCA can be used to give insight into this last category of foldable segments is illustrated, with examples matching known 3D structures. After reviewing the HCA principles, examples of short foldable segments are given, which often contain short linear motifs, typically matching hydrophobic clusters. These segments become ordered upon contact with partners, with secondary structure preferences generally corresponding to those observed in the 3D structures within the complexes. Such small foldable segments are sometimes larger than the segments of known 3D structures, including flanking hydrophobic clusters that may be critical for interaction specificity or regulation, as well as intervening sequences allowing fuzziness. Cases of larger conditionally disordered domains are also presented, with lower density in hydrophobic clusters than well-folded globular domains or with exposed hydrophobic patches, which are stabilized by interaction with partners.