Molecular dynamics computational analysis of protein interactions involving small heat shock proteins in human cardiac muscle tissue

Thesis

Small heat-shock proteins (sHSPs) are a class of molecular chaperones that are essential for the development and functionaility of human cardiac muscle tissue. Various types of sHSPs are expressed and localised within the sarcomere, the fundamental functional unit of striated muscle tissue, where they play a significant role in the regulation of sarcomeric proteins. To investigate the interactions between sHSPs and cardiac muscle proteins, we employ molecular dynamics (MD) simulations to model the behaviour of proteins and protein complexes.

Our studies initially examine the interaction between HspB1 and B5, which is essential for regulating the chaperoning functions of both sHSPs in cardiac muscle tissues. We confirm that HspB1 and B5 are capable of both self-assembly and co-assembly, resulting in the formation of hetero-oligomers. Notably, the affinities of the interactions between the $\alpha$-Crystallin domain (ACD) and the C-terminal domain (CTD) of HspB1 and HspB5 are not unifrom. Due to their varying interaction affinities, HspB1 and B5 can associate in different assembly states, which can modulate the chaperoning capabilities of both proteins. Based on binding energies and inter-residue distances calculated from MD simulations, we find that the CTD of HspB5 demonstrates greater binding affinities with ACDs compared to the CTD of HspB1. This discrepancy is attributed to the presence of additional arginine residues in the sequence, a conclusion that is corroborated by simulations of complexes involving the last common ancestral (LCA) protein of HspB1 and B5.

Next, we examine the regulatory effect of HspB7 on filamin C (FLNC), a protein that is essential for the formation of the sarcomeric backbone. The interaction between HspB7 and the FLNC domain 24 (FLNCd24) competes with the dimerisation of FLNCd24, a process that is vital for the functionality of filamin C as a linker of actin filaments. Additionally, our findings indicate that two phosphorylation sites within FLNCd24 modulate the equilibrium between two types of interactions. Specifically, phosphorylation of FLNCd24 at T2677 enhances homo-dimerisation, thereby surpassing the interaction between FLNCd24 and HspB7, which is critical for the normal function of FLNC. Conversely, pathogenic phosphorylation at Y2683 produces completely opposite effects.

Our studies also investigate the interaction between HspB5 and titin, with a particular emphasis on how this interaction modulates the mechanical properties of titin. Titin, a fibrous protein that serves as the structural backbone of the sarcomere, exhibits elastic properties and can be elongated in response to external forces. Our findings suggest that the binding of the immunoglobulin-like domain 26, an elastic region of titin, to the C-terminal segment of HspB5 enhances its resistance to mechanical stress. However, this regulatory effect is significantly reduced in the presence of the R157H mutation in HspB5.

Authors: Cao, G

• DOI: 10.5287/ora-kokj87erj
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: small heat shock proteins; sarcomeric proteins; protein-protein interactions; molecular dynamics
• Subjects: Molecular dynamics; Biochemistry