Abstract
Voltage-dependent cation channels are large heterooligomeric proteins. Heterologous expression of cDNAs encoding the alpha 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 alpha and beta subunits, tightly associated in a 400-kDa complex; although molecular details of the alpha-subunit proteins have been determined, little is known about the beta-subunit constituent. Proteolytic fragments of a beta subunit from bovine alpha-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 lambda gt10 cDNA library from bovine brain; a full-length clone subsequently isolated encodes a protein of 367 amino acids (M(r) approximately 40,983). It shows no significant homology with any known protein. Unlike the channels' alpha subunits, the hydropathy profile of this sequence failed to reveal transmembrane domains. Several consensus phosphorylation motifs are apparent and, accordingly, the beta 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 beta subunit being firmly associated on the inside of the membrane with alpha subunits, as speculated in a simplified model of these authentic K+ channels. Importantly, this first primary structure of a K(+)-channel beta subunit indicates that none of the cloned auxiliary proteins of voltage-dependent cation channels, unlike their alpha subunits, belong to a super-family of genes.