Quantitative description of co-assembly and evolution of small heat-shock proteins

Small heat-shock proteins (sHsps) are a family of molecular chaperones that are a part of the cellular proteostasis machinery. Many of them, such as human HspB1, HspB4 and HspB5 are polydisperse–they form a large range of stoichiometries. Some are also known to co-assemble, forming a large variety of sub-stoichiometries, with possible implications for their function. In this work, we first develop theoretical models for studying the co-assembly between both monodisperse as well as polydisperse proteins using statistical mechanics and chemical kinetics, respectively. We then briefly apply the statistical-mechanical model on a system of two co-assembling monodisperse sHsps. We then use two orthogonal biophysical techniques, native mass spectrometry (MS) and mass photometry (MP), to study the homo-oligomerisation of the two most abundant, and highly polydisperse, human sHsps–HspB1 and HspB5. Next, we use MS and MP to study highly heterogeneous mixtures of these two proteins on both the level of stoichiometries as well as the sub-stoichiometry level. We next use the model we have developed and apply it to our mixing data to obtain both the complete description of these co-assemblies as well as to extract any energy biases in these mixtures, which we then probe experimentally using MS, MP and molecular dynamics modelling. Finally, we study the co-assembly of HspB1, HspB5 and HspB6 between different vertebrate species and mention that some HspB1 and hspB6 orthologs either completely lost, or are in the process of losing, their polydispersity. Curiously, HspB5 orthologs still seem to be polydisperse, which might be due to the functional role of polydispersity in the eye lens, where HspB5 is highly expressed.