Channelpedia

BKβ2

Description: potassium large conductance calcium-activated channel, subfamily M, beta member 2
Gene: Kcnmb2
Alias: Kcmb2, Kcnmb2, BKB2

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Introduction

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


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Gene

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])

Species NCBI gene ID Chromosome Position
Human 10242 3 307993
Mouse 72413 3 298308
Rat 294961 2 305379

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Transcript

Species NCBI accession Length (nt)
Human NM_181361.3 2543
Mouse NM_028231.2 2947
Rat NM_176861.1 708

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Protein Isoforms

Species Uniprot ID Length (aa)
Human Q9Y691 235
Mouse Q9CZM9 235
Rat Q811Q0 235

Isoforms

Transcript
Length (nt)
Protein
Length (aa)
Variant
Isoform

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Post-Translational Modifications

PTM
Position
Type

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Structure

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])

BKβ2 predicted AlphaFold size

Species Area (Å2) Reference
Human 4254.10 source
Mouse 4348.65 source
Rat 4150.55 source

Methodology for AlphaFold size prediction and disclaimer are available here


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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]).


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Expression and Distribution

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


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Function

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]).


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Interaction

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]).


References

540

Orio P et al. New disguises for an old channel: MaxiK channel beta-subunits.
News Physiol. Sci., 2002 Aug , 17 (156-61).

Vergara C et al. Calcium-activated potassium channels.
Curr. Opin. Neurobiol., 1998 Jun , 8 (321-9).

Wallner M et al. Molecular basis of fast inactivation in voltage and Ca2+-activated K+ channels: a transmembrane beta-subunit homolog.
Proc. Natl. Acad. Sci. U.S.A., 1999 Mar 30 , 96 (4137-42).


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Credits

To cite this page: [Contributors] Channelpedia https://channelpedia.epfl.ch/wikipages/73/ , accessed on 2024 Dec 12



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