Palmitoylation of the β4-Subunit Regulates Surface Expression of Large Conductance Calcium-activated Potassium Channel Splice Variants

Regulatory beta-subunits of large conductance calcium- and voltage-activated potassium (BK) channels play an important role in generating functional diversity and control of cell surface expression of the pore forming alpha-subunits. However, in contrast to alpha-subunits, the role of reversible post-translational modification of intracellular residues on beta-subunit function is largely unknown. Here we demonstrate that the human beta 4-subunit is S-acylated (palmitoylated) on a juxtamembrane cysteine residue (Cys-193) in the intracellular C terminus of the regulatory beta-subunit. beta 4-Subunit palmitoylation is important for cell surface expression and endoplasmic reticulum (ER) exit of the beta 4-subunit alone. Importantly, palmitoylated beta 4-subunits promote the ER exit and surface expression of the pore-forming alpha-subunit, whereas beta 4-subunits that cannot be palmitoylated do not increase ER exit or surface expression of alpha-subunits. Strikingly, however, this palmitoylation-and beta 4-dependent enhancement of alpha-subunit surface expression was only observed in alpha-subunits that contain a putative trafficking motif (... REVEDEC) at the very C terminus of the alpha-subunit. Engineering this trafficking motif to other C-terminal alpha-subunit splice variants results in alpha-subunits with reduced surface expression that can be rescued by palmitoylated, but not depalmitoylated, beta 4-subunits. Our data reveal a novel mechanism by which palmitoylated beta 4-subunit controls surface expression of BK channels through masking of a trafficking motif in the C terminus of the alpha-subunit. As palmitoylation is dynamic, this mechanism would allow precise control of specific splice variants to the cell surface. Our data provide new insights into how complex interplay between the repertoire of post-transcriptional and post-translational mechanisms controls cell surface expression of BK channels.