Despite their prevalence in nature and as drug and therapeutic targets, many membrane proteins continue to evade structure determination by X-ray crystallography and NMR. The combination of site-directed spin labeling with electron paramagnetic resonance (SDSL-EPR) is becoming an increasingly popular method for the structural characterization of membrane proteins due to the relative ease with which they can be studied. However, SDSL-EPR does not yield high-resolution structures directly. The current proposal describes a new method, ROSETTAEPR, to overcome this obstacle. ROSETTAEPR will be a toolkit in which distance and accessibility restraints determined by EPR will be combined with Monte Carlo-based computational methods for the de novo structure prediction of proteins. After developing knowledge-based potentials derived from EPR experimental data, it will be benchmarked on proteins of known structure using both simulated and real EPR data. In addition, ROSETTAEPR and EPR experimental distance and accessibility data will be used to determine the LeuT apo, Na+, and Na+/leucine bound structural intermediates involved in leucine transport. LeuT is a bacterial homolog of the neurotransmitter sodium symporter (NSS) protein family, which includes the dopamine, serotonin, and norepinephrine transporters. While there are no high-resolution structures of the NSS transporters, extracellular-facing substrate-bound conformations of LeuT have been determined by X-ray crystallography. However, the current structures are static snapshots of the LeuT transport cycle;furthermore, they are believed to have been captured in a potentially inhibited form. Therefore, EPR spectroscopy has been employed to shed light on the dynamics of the protein. It was found that Na+ binding causes an increase in protein flexibility in the extracellular loops and hydration of the substrate permeation pathway, while subsequent binding of leucine causes the extracellular vestibule to close and become rigid. ROSETTAEPR will allow for the high-resolution structural elucidation of these intermediates based on low-resolution EPR data.
The dysfunction of neurotransmitter sodium symporter (NSS) proteins is a common characteristic of central nervous system (CNS) diseases, such as depression, anxiety, obsessive compulsive disorder (OCD), and epilepsy. Understanding how these proteins function on a structural level will aid in the development of new, more effective therapeutic agents that specifically target neural processes underlying mood, reward, and locomotion.