Endocytic trafficking signals in KCNMB2 regulate surface expression of a large conductance voltage and Ca2+-activated K+ channel

Large conductance voltage and calcium-activated K+ channels play critical roles in neuronal excitability and vascular tone. Previously, we showed that coexpression of the transmembrane β2 subunit, KCNMB2, with the human pore-forming α subunit of the large conductance voltage and Ca2+-activated K+ channel (hSlo) yields inactivating currents similar to those observed in hippocampal neurons [Hicks GA, Marrion NV (1998) Ca2+-dependent inactivation of large conductance Ca2+-activated K+ (BK) channels in rat hippocampal neurones produced by pore block from an associated particle. J Physiol (Lond) 508 (Pt 3):721–734; Wallner M, Meera P, Toro L (1999b) Molecular basis of fast inactivation in voltage and Ca2+-activated K+ channels: A transmembrane β-subunit homolog. Proc Natl Acad Sci U S A 96:4137–4142]. Herein, we report that coexpression of β2 subunit with hSlo can also modulate hSlo surface expression levels in HEK293T cells. We found that, when expressed alone, β2 subunit appears to reach the plasma membrane but also displays a distinct intracellular punctuated pattern that resembles endosomal compartments. β2 Subunit coexpression with hSlo causes two biological effects: i) a shift of hSlo’s intracellular expression pattern from a relatively diffuse to a distinct punctated cytoplasmic distribution overlapping β2 expression; and ii) a decrease of hSlo surface expression that surpassed an observed small decrease in total hSlo expression levels. β2 Site-directed mutagenesis studies revealed two putative endocytic signals at the C-terminus of β2 that can control expression levels of hSlo. In contrast, a β2 N-terminal consensus endocytic signal had no effect on hSlo expression levels. Thus, β2 subunit not only can influence hSlo currents but also has the ability to limit hSlo surface expression levels via an endocytic mechanism. This new mode of β2 modulation of hSlo may depend on particular coregulatory mechanisms in different cell types.