Membrane transport mechanisms

Movement of molecules across cell membranes is perquisite for all living organisms. To facilitate and regulate this vital cellular process, organism have evolved specialized membrane proteins such as transporter, channels, or complex secretion systems. During my doctoral and postdoctoral research, I investigated some of the molecular mechanisms that govern the secretion of virulence effector toxins via the Type III secretion system, the broad substrate selectivity in the SLC22 family of transporters, and the receptor-mediated translocation of botulinum neurotoxin.

Considering that protein unfolding, which is necessary for type-III secretion system (T3SS)-mediated secretion, is an energetically unfavorable process, it remains largely unclear how pathogenic bacteria manage to unfold and secrete hundreds of toxic proteins within seconds. We show ExoY and other T3SS effector proteins have evolved to minimize the energetic cost of unfolding by acquiring key structural features associated with partially unfolded proteins. These features include abundance of disordered regions, weak hydrophobic core, and inherent stereo-chemical frustrations in their folded core, allowing them to be efficiently unfolded by proton-concentration gradient (~pH 5.8-6.0, generated by proton-motive force) and T3SS ATPase for secretion through narrow conduct. 

The SLC22 family of transporters comprising of organic cation transporters (OCTs), organic zwitterion/cation transporters (OCTNs), and organic anion transporters (OATs), mediated uptake of nutrients, drugs, xenobiotics, and endogenous compounds and perform homeostatic functions. We solve cryo-EM structures of human OCT3, providing basis its broad substrate selectivity, and specific inhibition by endogenous and exogenous molecules.

Synaptic vesicle glycoprotein 2 (SV2) are non canonical members of SLC22 family of transporters. While present almost every secretory vesicles, the function of these membrane proteins as transporter is unclear. However, these proteins are important receptor for antiepileptic drugs (e.g. Leveretictum) and host receptor for Botulinum neurotoxins (shortly known as Botox).  Botulinum neurotoxins A1 (BoNT/A1) is the most potent biological toxin and the second most widely used therapeutic protein. These toxin use synaptic vesicle glycoprotein 2 (SV2) to bind and enter host neuronal cells. However, the molecular mechanisms underlying its recognition and regulation during cytosolic translocation remain unclear.