Functionally-coupled ion channels begin co-assembling at the start of their synthesis

Calcium binding to BK channels lowers BK activation threshold,
substantiating functional coupling with calcium-permeable channels. This
coupling requires close proximity between different channel types, and the
formation of BK–CaV1.3 hetero-clusters at nanometer distances exemplifies
this unique organization. To investigate the structural basis of this
interaction, we tested the hypothesis that BK and CaV1.3 channels assemble
before their insertion into the plasma membrane. Our approach incorporated
four strategies: (1) detecting interactions between BK and CaV1.3 proteins
inside the cell, (2) identifying membrane compartments where intracellular
hetero-clusters reside, (3) measuring the proximity of their mRNAs, and
(4) assessing protein interactions at the plasma membrane during early
translation. These analyses revealed that a subset of BK and CaV1.3
transcripts are spatially close in micro-translational complexes, and
their newly synthesized proteins associate within the endoplasmic
reticulum (ER) and Golgi. Comparisons with other proteins, transcripts,
and randomized localization models support the conclusion that BK and
CaV1.3 hetero-clusters form before their insertion at the plasma membrane.