Title
Mechanical Softening of a Small Ubiquitin-related Modifier Protein (SUMO2) Due to Temperature Induced Flexibility at the Core
Author
Shrabasti Bhattacharya, Sri Rama Koti Ainavarapu
Year
2018
Journal
The Journal of Physical Chemistry B
Abstract
Despite the growing interest in the thermal softening of proteins, the mechanistic details of it are far from understood. β-grasp proteins have globular shape with compact structure and they are mechanically resilient. The β-clamp or mechanical clamp in them formed by the interactions between the terminal β strands is generally associated with the protein mechanical resistance. Although previous studies showed that temperature can perturb the protein mechanical stability, the structural changes leading to the lowered mechanical resistance are not known. Here, we investigated the temperature-dependent mechanical stability of small ubiquitin-related modifier 2 (SUMO2) using single-molecule force spectroscopy (SMFS) and the corresponding conformational changes using ensemble experiments. SMFS studies on the polyprotein of SUMO2 estimate a decrease in the spring constant of the protein from 4.50 N/m to 1.35 N/m upon increasing the temperature from 5-45 0C. Interestingly, near-UV circular dichroism spectroscopy reveals a decrease in tertiary structure content while the overall secondary structure of the protein remains unchanged. Steady-state fluorescence and quenching studies on SUMO2 with a tryptophan mutation at the core (F60W) show that the nonpolar environment of the tryptophan is unchanged and the protein core is inaccessible to the bulk solvent, in the same temperature range. We attribute the thermal softening observed in AFM experiments to the reduction in tertiary structure of SUMO2. Our results provide evidence for the importance of the intramolecular interactions at the protein core along with the β- or mechanical clamp in providing the mechanical resistance as well as in modulating the protein stiffness.
Instrument
J-810
Keywords
Circular dichroism, Secondary structure, Tertiary structure, Thermal stability, Thermodynamics, Protein folding, Biochemistry