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BKB2

potassium large conductance calcium-activated channel, subfamily M, beta member 2
Synonyms: Kcmb2 Kcnmb2. Symbol: Kcnmb2

Introductions


Until 1999, only one beta subunit of BK (MaxiK) channels was known at the molecular level. This subunit (now known as the Beta-1-subunit) accounted mainly for MaxiK properties in vascular smooth muscle cells (VSMCs), where it is highly expressed. In the following years, three more beta subunits have been cloned and characterized. (Orio [540])

KCNMB2 (also known as MGC22431) encodes the beta2 subunit of BK (MaxiK) channels. MaxiK channels are large conductance, voltage and calcium-sensitive potassium channels which are fundamental to the control of smooth muscle tone and neuronal excitability. MaxiK channels can be formed by 2 subunits: the pore-forming alpha subunit and the modulatory beta subunit. b2 is an auxiliary beta subunit which decreases the activation time of MaxiK alpha subunit currents. Two variants encoding the same protein have been found for this gene.

http://www.ncbi.nlm.nih.gov/gene/10242

Genes


Sequence similarities are major between b1-b2 and b2-b3, respectively. b4 is the most distantly related of all b-subunits. b-Subunit orthologs have not been described in Drosophila or in the worm C. elegans, suggesting that this protein is a “novel” acquisition in evolution. (Orio [540])

Kcnmb2 : potassium large conductance calcium-activated channel, subfamily M, beta member 2

RGD ID Chromosome Position Species
631398 2 118363702-118401375 Rat
1331983 3 31801625-32099102 Mouse
1343870 3 178254224-178562217 Human

Transcripts


Acc No Sequence Length Source
NM_176861 NCBI
NM_028231 NCBI
NM_181361 NCBI
NM_005832 NCBI

Ontologies


Accession Name Definition Evidence
GO:0016021 integral to membrane Penetrating at least one phospholipid bilayer of a membrane. May also refer to the state of being buried in the bilayer with no exposure outside the bilayer. When used to describe a protein, indicates that all or part of the peptide sequence is embedded in the membrane. IEA
GO:0016020 membrane Double layer of lipid molecules that encloses all cells, and, in eukaryotes, many organelles; may be a single or double lipid bilayer; also includes associated proteins. IEA
GO:0008076 voltage-gated potassium channel complex A protein complex that forms a transmembrane channel through which potassium ions may cross a cell membrane in response to changes in membrane potential. IEA
GO:0005887 integral to plasma membrane Penetrating at least one phospholipid bilayer of a plasma membrane. May also refer to the state of being buried in the bilayer with no exposure outside the bilayer. IEA

Interactions


Coexpression of alpha- and b2-subunits produces inactivating currents, such as those seen in some chromaffin cells ( Orio [540]). Removal of the NH2-terminal domain, either by trypsin or molecular biology techniques, results in a b-subunit that does not inactivate the channel, so the currents are sustained and more suitable for kinetic and Ca2+ sensitivity comparisons. In contrast to the b1 subunit, the b2 subunit confers low CTX affinity compared with channels formed only by alpha-subunits (Xia [1182]).

Proteins


Structures


The topology of alpha and beta subunits that make up BK channels can be seen in fig. 1 of Orio [540]. The channel is formed by 4 alpha-subunits and probably 4 beta subunits. Regulatory beta subunits share a putative membrane topology, with two transmembrane segments connected by a 120- residue extracellular “loop” and with NH2 and COOH termi- nals oriented toward the cytoplasm (Fig. 1 [540]). The loop has three or four putative glycosylation sites. Four beta subunits have been cloned in mammals (Brenner [1181], Knaus [1167], Uebele [1179], Xia [1182]). The most notorious difference from b2 to the b1-subunit is an NH2- terminal domain that contains a hydrophobic region followed by positively charged residues. This type of sequence is characteristic of inactivation peptides that can occlude the conduction pathway of Shaker K+ and of MaxiK channels. (Orio [540])

Distributions


Expressions


b2 subunit is expressed preferentially in chromaffin cells and brain (Xia [11]](#a1182)).

Functionals


In In rat chromaffin cells, there is a cell population that has inactivating MaxiK currents (MaxiKi) and a population with noninactivating MaxiK currents (MaxiKs) (Vergara [1100]). In the MaxiKi cells, the b2-subunit is expressed. As a consequence, MaxiK channels in these cells show rapid inactivation and slow deactivation kinetics (Xia [1182]). Because of the much slower deactivation of the MaxiK channel, the afterhyperpolarization phase is prolonged. This relieves Na+ channel inactivation and allows repetitive firing. Thus while MaxiKi are tonically firing cells, MaxiKs are phasically firing cells (Fig. 3 in [540], Solaro [1183]).

Kinetics


Ca2+ sensitivity and gating kinetics of channels formed by alpha- and b2-subunits are similar to those of channels formed by alpha- and b1-subunits (Xia [1182]).

Models


References


[1174 : 19204046]
[1175 : 17521822]
[1176 : 16446507]
[1177 : 12566540]
[540 : 12136044]
[1178 : 11517232]
[1180 : 10097176]
[1181 : 10692449]
[1167 : 8006036]
[1179 : 10766764]
[1182 : 10377337]
[1183 : 7545225]
[1100 : 9687354]

Credits