Primary Structure of a β Subunit of α-Dendrotoxin-Sensitive K+Channels from Bovine Brain

Voltage-dependent cation channels are large heterooligomeric proteins. Heterologous expression of cDNAs encoding the α subunits alone of K+, Na+, or Ca2+channels produces functional multimeric proteins; however, coexpression of those for the latter two with their auxiliary proteins causes dramatic changes in the resultant membrane currents. Fast-activating, voltage-sensitive K+channels from brain contain four α and β subunits, tightly associated in a 400-kDa complex; although molecular details of the α-subunit proteins have been determined, little is known about the β-subunit constituent. Proteolytic fragments of a β subunit from bovine α-dendrotoxin-sensitive neuronal K+channels yielded nine different sequences. In the polymerase chain reaction, primers corresponding to two of these peptides amplified a 329-base-pair fragment in a λ gt10 cDNA library from bovine brain; a full-length clone subsequently isolated encodes a protein of 367 amino acids (Mr≈ 40,983). It shows no significant homology with any known protein. Unlike the channels' α subunits, the hydropathy profile of this sequence failed to reveal transmembrane domains. Several consensus phosphorylation motifs are apparent and, accordingly, the β subunit could be phosphorylated in the intact K+channels. These results, including the absence of a leader sequence and N-glycosylation, are consistent with the β subunit being firmly associated on the inside of the membrane with α subunits, as speculated in a simplified model of these authentic K+channels. Importantly, this first primary structure of a K+-channel β subunit indicates that none of the cloned auxiliary proteins of voltage-dependent cation channels, unlike their α subunits, belong to a superfamily of genes.