Structural Studies of Septin2G Amyloid Fibrils

Abstract

Septins are proteins from the GTP-binding family and participate in cell division cycle performing functions such as secretion and cytoskeletal division. They can also be found in neurodegenerative conditions as Alzheimer's and Parkinson's diseases, forming highly organized fiber-like aggregates known as amyloids. In this work, we used small angle x-ray scattering (SAXS) to investigate the formation and time evolution of septins aggregates under the influence of temperature and concentration.The SAXS measurements were performed with the GTPase domain of human Septin 2 (SEPT2G) at 0.5 and 1 mg/mL and temperatures from 4 to 45°C. At 4°C, our results demonstrate that SEPT2G is self-aggregated as a dimer at 0.5mg/mL, whereas dimers coexist with cylinder-like aggregates (36 nm-long and 12 nm-cross section) at 1mg/mL. At this temperature, the protein does not evolve over one hour of observation.As the temperature was increased to 15°C we verified that, initially coexisting with the protein dimer and cylinders, a small amount of larger aggregates is also present in solution. However, the number of very large aggregates increases with time concomitantly with the decrease of cylinder amount in the solution. Analyzing the samples at 37°C it's not possible to observe cylinders anymore and the amount of dimers decreases from 50% to 20% in less than 1 hour. For 45°C this effect is even more accentuated: the percentage of dimers is only 6% in solution.In conclusion, our results showed the coexistence of dimers of SEPT2G with small fibers and larger aggregates in solution that evolve not only with concentration and temperature but also with time.This work is supported by FAPESP and CNPq.The authors thank the LNLS SAXS beam line staff.