Absence of the beta subunit (cchb1) of the skeletal muscle dihydropyridine receptor alters expression of the alpha 1 subunit and eliminates excitation-contraction coupling.
The multisubunit (alpha 1s, alpha 2/delta, beta 1, and gamma) skeletal muscle dihydropyridine receptor transduces transverse tubule membrane depolarization into release of Ca2+ from the sarcoplasmic reticulum, and also acts as an L-type Ca2+ channel. The alpha 1s subunit contains the voltage sensor and channel pore, the kinetics of which are modified by the other subunits. To determine the role of the beta 1 subunit in channel activity and excitation-contraction coupling we have used gene targeting to inactivate the beta 1 gene. beta 1-null mice die at birth from asphyxia. Electrical stimulation of beta 1-null muscle fails to induce twitches, however, contractures are induced by caffeine. In isolated beta 1-null myotubes, action potentials are normal, but fail to elicit a Ca2+ transient. L-type Ca2+ current is decreased 10- to 20-fold in the beta 1-null cells compared with littermate controls. Immunohistochemistry of cultured myotubes shows that not only is the beta 1 subunit absent, but the amount of alpha 1s in the membrane also is undetectable. In contrast, the beta 1 subunit is localized appropriately in dysgenic, mdg/mdg, (alpha 1s-null) cells. Therefore, the beta 1 subunit may not only play an important role in the transport/insertion of the alpha 1s subunit into the membrane, but may be vital for the targeting of the muscle dihydropyridine receptor complex to the transverse tubule/sarcoplasmic reticulum junction.
Gregg
R G
RG
Waisman Center, University of Wisconsin, Madison 53705, USA. gregg@waisman.wisc.edu
Messing
A
A
Strube
C
C
Beurg
M
M
Moss
R
R
Behan
M
M
Sukhareva
M
M
Haynes
S
S
Powell
J A
JA
Coronado
R
R
Powers
P A
PA
D000200:Action Potentials / Q000502:physiology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000096:biosynthesis* / Q000235:genetics* / Q000502:physiology; D020746:Calcium Channels, L-Type; D003001:Cloning, Molecular; D004622:Embryo, Mammalian; D015698:Genomic Library; D046911:Macromolecular Substances; D051379:Mice; D018345:Mice, Knockout; D009119:Muscle Contraction; D018482:Muscle, Skeletal / Q000502:physiology*; D009210:Myofibrils / Q000502:physiology / Q000648:ultrastructure; D011994:Recombinant Proteins / Q000096:biosynthesis; D012519:Sarcoplasmic Reticulum / Q000378:metabolism
D015220:Calcium Channels; D020746:Calcium Channels, L-Type; D046911:Macromolecular Substances; D011994:Recombinant Proteins; D002118:Calcium