Ion Channel-Transporter Complexes
- Author(s): Neverisky, Daniel Loerch
- Advisor(s): Abbott, Geoffrey W
- et al.
All living cells require membrane proteins that act as conduits for the regulated transport of ions, solutes and other small molecules across the cell membrane. Here, we discuss the recent discovery of two critical classes of such membrane proteins forming co-regulatory complexes in vivo and in vitro; namely, voltage-gated potassium channels and sodium-dependent myo-inositol transporters of the SLC5A family. These ion channels provide a pore that permits rapid, highly selective, and tightly regulated movement of ions down their electrochemical gradient, providing critical effects on cellular excitability and action potential termination. By contrast, the transporters move myo-inositol by coupling uphill movement of the substrate to downhill movement of another ion, sodium, to participate in the phosphatidylinositol signaling pathway. It is well known that proteins in each of these classes work in concert with members of the other classes to ensure, for example, ion homeostasis, ion secretion, and restoration of ion balance following action potentials. More recently, evidence is emerging of direct physical interaction between true ion channels and transporters, and regulation of ion channel activity and cellular excitability by SLC5A transporter SMIT1. Here, we describe discovery and functional characteristics of novel members of a new class of the macromolecular channel-transporter complexes that we term “chansporters”, and their diverse effect on each constituent’s function.
In the first part of this dissertation, we discuss our initial discovery of chansporter complexes – that KCNQ1 and SMIT1 reciprocally augment each other’s function and that KCNQ1-associating β-subunit KCNE2 drastically alters function of the complexed transporter as well as channel.
In the second part, we discuss the discovery of the contrasting effect of KCNQ2 on SMIT1 and SMIT2 – unlike KCNQ1, KCNQ2 strongly inhibits myo-inositol uptake by the transporters. This effect is demonstrated to be associated with a matched reduction in transporter surface expression, but may also be altered by cellular depolarization and select mutations, suggesting that channel conformation or state may also play a role in altering transporter function. Last, co-immunoprecipitations of truncated KCNQ2 proteins and SMIT1 demonstrate that the pore-forming S5-S6 region of KCNQ2 is required for interaction with SMIT1.