Competitive modulation of K1.2 gating by LMAN2 and Slc7a5.

K1.2 is a prominent ion channel in the CNS, where it regulates neuronal excitability. K1.2 structure and function are well understood, but there is less consensus on mechanisms of regulation of K1.2 and other potassium channels by auxiliary proteins. We previously identified novel regulators of K1.2 by a mass spectrometry approach. The neutral amino acid transporter Slc7a5 causes a dramatic hyperpolarizing shift of channel activation. In contrast, the transmembrane lectin LMAN2 is a recently identified candidate regulator that has the opposite effect on gating: large depolarizing voltages are required to activate K1.2 channels co-expressed with LMAN2. In this study, we characterized the functional interaction between LMAN2 and Slc7a5 on K1.2 gating properties and identified key structural elements that underlie sensitivity to each regulator. When LMAN2 and Slc7a5 are expressed together, K1.2 activation exhibits a bi-modal voltage-dependence, suggesting two distinct populations of channels regulated either by LMAN2 or Slc7a5, but not both. Using a K1.2:1.5 chimeric approach, we identified specific regions between the S1 to S3 segments of the voltage sensing domain (VSD) that are distinct for either Slc7a5 or LMAN2 sensitivity. By replacing either segment with sequence from K1.5, modulation by the corresponding regulator was selectively abolished. These results suggest that Slc7a5 and LMAN2 compete for interaction with the K1.2 voltage sensor, leading to complex voltage-dependence of channel activity when both regulators are present in the cell.