The Connection Between Thermo Stability And Expression Level Of Thermophilic Protein Complexes

Protein complexes are major forms of protein-protein interactions, implement essential biological functions and thus need stable quaternary structures. The thermostability of a protein complex is generally determined by the interaction strength between its subunits and reflected in the contact interface. Previous studies have revealed that the properties of subunit interface of protein complexes in thermophiles are significantly different from those in mesophiles and that the proportion of highly expressed proteins in prokaryotes is related with their optimal growth temperature (OGT). However, the connection between the thermostability of a protein complex and its gene expression level is still unknown.In this thesis, based on the3D structure information from Protein Data Bank (PDB), we analyzed the subunit interface features of protein complexes in two thermophilic bacteria and three mesophilic bacteria by calculating the interface size, the binding energy and the chemical properties of interface. Moreover, we classified the protein complexes according to their expression levels and compare the differences of interface features between highly expressed and lowly expressed protein complexes. Finally, by running a computer simulation for protein complex evolution, we explored the generation mechanism of the observed difference under a selection pressure of temperature.The results show that the highly expressed protein complexes have larger subunit interfaces and higher thermostability than lowly expressed ones in thermophilic bacteria rather than mesophilic bacteria. The simulation for protein complex evolution demonstrated that the evolution of thermostability is faster in highly expressed protein complexes than that in lowly expressed ones under selection pressure of high temperature, while such a relationship is absent in the evolution simulation under normal temperature. We therefore infer that the difference of the connection between thermostability and expression level of protein complexes in the organisms with different OGT is caused by the environment temperature pressure. Our work furthers the understanding for the thermal adaptation mechanisms of protein complexes.