Originally published In Press as doi:10.1074/jbc.M411501200 on November 9, 2004
J. Biol. Chem., Vol. 280, Issue 3, 2229-2237, January 21, 2005
The Ca2+ Channel 
2
-1 Subunit Determines Ca2+ Current Kinetics in Skeletal Muscle but Not Targeting of 1S or Excitation-Contraction Coupling*
Gerald J. Obermair
,
Gerlinde Kugler,
Sabine Baumgartner,
Petronel Tuluc,
Manfred Grabner
, and
Bernhard E. Flucher¶
From the
Department of Physiology and Medical Physics and the Department of Medical Genetics, Molecular and Clinical Pharmacology, Innsbruck Medical University, 6020 Innsbruck, Austria
Auxiliary channel subunits regulate membrane expression and modulate current properties of voltage-activated Ca2+ channels and thus are involved in numerous important cell functions, including muscle contraction. Whereas the importance of the 
1S, 
1a, and 
Ca2+ channel subunits in skeletal muscle has been determined by using null-mutant mice, the role of the 2
-1 subunit in skeletal muscle is still elusive. We addressed this question by small interfering RNA silencing of 2-1 in reconstituted dysgenic (1S-null) myotubes and in BC3H1 skeletal muscle cells. Immunofluorescence labeling of the 1S and 2-1 subunits and whole cell patch clamp recordings demonstrated that triad targeting and functional expression of the skeletal muscle Ca2+ channel were not compromised by the depletion of the 2-1 subunit. The amplitudes and voltage dependences of L-type Ca2+ currents and of the depolarization-induced Ca2+ transients were identical in control and in 2-1-depleted muscle cells. However, 2-1 depletion significantly accelerated the current kinetics, most likely by the conversion of slowly activating into fast activating Ca2+ channels. Reverse transcription-PCR analysis indicated that 2-1 is the exclusive isoform expressed in differentiated BC3H1 cells and that depletion of 2-1 was not compensated by the up-regulation of any other 2 isoform. Thus, in skeletal muscle the Ca2+ channel 2-1 subunit functions as a major determinant of the characteristic slow L-type Ca2+ current kinetics. However, this subunit is not essential for targeting of Ca2+ channels or for their primary physiological role in activating skeletal muscle excitation-contraction coupling.
Received for publication, October 8, 2004
, and in revised form, November 4, 2004.
The nucleotide sequence(s) reported in this paper has been submitted to the DDBJ/GenBankTM/EBI Data Bank with accession number(s) TPA BK005394.
* This work was supported by grants from the Austrian Science Fund and Austrian National Bank Grants P15338-B05, P16532-B05 (to B. E. F.), and P16098-B11 (to M. G.), and by European Commission Grant HPRN-CT-2002-00331 (to B. E. F.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
The on-line version of this article (available at http://www.jbc.org) contains Supplements 1–4.
¶ To whom correspondence should be addressed: Dept. of Physiology and Medical Physics, Innsbruck Medical University, Fritz-Pregl-Str. 3, A-6020 Innsbruck, Austria. Tel.: 43-512-507-3787; Fax: 43-512-507-2836; E-mail: bernhard.e.flucher{at}uibk.ac.at.
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Copyright © 2005 by the American Society for Biochemistry and Molecular Biology.
