Functionally-Coupled Ion Channels Begin Co-assembling at the Start of Their Synthesis

Calcium binding to BK channels lowers their activation threshold. Hence, BK channels are functionally coupled to calcium-permeable channels. This functional coupling requires proximity of two different types of channels. Formation of an ensemble at nanometer distances between BK and CaV1.3 channels exemplifies this unique organization. We investigated the mechanism underlying their structural organization, testing the hypothesis that the assembly between BK and CaV1.3 channels is formed before their insertion at the plasma membrane. Our design used four approaches: 1) to detect the interaction between BK and CaV1.3 proteins inside the cell, 2) to identify the membranes where intracellular ensembles reside, 3) to measure the proximity between their mRNAs, and 4) to look for protein interaction at the plasma membrane soon after the start of their translation. These measures showed that a percentage of transcripts for BK and CaV1.3 channels are close and that the newly synthesized proteins interact through the endoplasmic reticulum and Golgi. Comparisons were made with other proteins and transcripts, as well as with predictions of randomized localizations of BK channels. We found evidence of assembly between BK and CaV1.3 ensembles before their insertion at the plasma membrane.