Using tetrapeptide GSSS and β-catenin17-48 to study the effects of phosphoserine on polypeptide conformational ensembles

Phosphorylation is one of the most common post-translational modifications and is known to have roles in protein transport, localization, function, and stability. Studying both the phosphorylated and nonphosphorylated states of a protein is important as they can be correlated to health and disease states, such as in Alzheimer's and Parkinson's disease. β-catenin is a protein that has two independent functions: cell-cell adhesion and transcriptional regulation, and when there is a change in the balance of these roles, β-catenin acts as an oncogene. It is thought that the phosphorylation state of the unstructured N-terminal region plays an important role in regulating the function and localization of β-catenin. Additionally, mutations at Ser33 and Ser37 phosphorylation sites have been associated with several types of cancer including liver, ovarian, and colorectal cancer. The addition of a large, negatively charged group on the serine sidechain is expected to strongly perturb local electric fields, thus influencing backbone conformational sampling and propensity for hydrogen bonding. As such, dipole-dipole interactions are expected to feature prominently in these systems and are best suited to be investigated with an explicitly polarizable force field. Parameters for phosphoserine were recently developed for the Drude polarizable force field. To validate these parameters and study a biologically relevant system, we performed both unbiased and enhanced-sampling molecular dynamics simulations of the phosphorylated and nonphosphorylated states of model tetrapeptide GSSS and a peptide derived from the N-terminal region of β-catenin, including residues 17-48 (β-catenin17-48). We calculated 3JHNHα-couplings to compare against experimental data and assessed the effect of phosphorylation on secondary structure content, peptide-bond dipole moments, and the electric fields acting around the phosphorylation sites. In doing so, we seek to relate electronic structure with polypeptide conformational dynamics.