Correct partner makes the difference: Septin G-interface plays a critical role in amyloid formation.

Septins are members of a group of GTP-binding proteins highly conserved in eukaryotes, being linked to diverse cell processes, such as cytokinesis and membrane association. On the other hand, the malfunction of septins is linked to several pathological processes including neurodegeneration and oncogenesis. Septins interact with each other forming heterocomplexes that polymerize in filaments. Two types of interface between septins alternate along the filament: the G-interface (involving the GTP binding sites), and the NC-interface. This work focuses on the physiological G-interface of SEPT2, used in the SEPT6G-SEPT2G heterodimer assembly, to verify the impact of this interaction on the thermostability and amyloid formation. We found that the SEPT6G-SEPT2G moves to an irreversible state with the ability to bind thioflavin-T at high temperatures, suggesting its amyloid-like nature. Noteworthy, this takes place at a higher temperature than the one observed to the single septins, showing greater thermal/structural stability. Taken together, our results show that in the absence of the partners, the septin becomes unstable and susceptible to amyloid aggregation/formation even in physiological temperatures, and the G-interface appears to have a critical role in this process.

Membrane damage efficiency of phenothiazinium photosensitizers.

Structure-activity relationships have been widely reported for porphyrin and phthalocyanine photosensitizers, but not for phenothiazinium derivatives. Here, four phenothiazinium salts (methylene blue, toluidine blue O, 1,9-dimethyl methylene blue and the pentacyclic derivative DO15) were used to investigate how the ability to damage membranes is affected by membrane/solution partition, photophysical properties and tendency to aggregation of the photosensitizer. These two latter aspects were studied both in isotropic solutions and in membranes. Membrane damage was assessed by leakage of a fluorescent probe entrapped in liposomes and by generation of thiobarbituric acid-reactive species (TBARS), while structural changes at the lipid bilayer were detected by small-angle X-ray scattering. We observed that all compounds had similar singlet-oxygen quantum yields in ethanol, but only the photosensitizers that had higher membrane/solution partition (1,9-dimethyl methylene blue and DO15, the latter having the higher value) could permeabilize the lipid bilayer. Moreover, of these two photosensitizers, only DO15 altered membrane structure, a result that was attributed to its destabilization of higher order aggregates, generation of higher amounts of singlet oxygen within the membranes and effective electron-transfer reaction within its dimers. We concluded that membrane-based protocols can provide a better insight on the photodynamic efficiency of the photosensitizer.