The II-III loop of the skeletal muscle dihydropyridine receptor is responsible for the Bi-directional coupling with the ryanodine receptor.
The dihydropyridine receptor (DHPR) in the skeletal muscle plasmalemma functions as both voltage-gated Ca(2+) channel and voltage sensor for excitation-contraction (EC) coupling. As voltage sensor, the DHPR regulates intracellular Ca(2+) release via the skeletal isoform of the ryanodine receptor (RyR-1). Interaction with RyR-1 also feeds back to increase the Ca(2+) current mediated by the DHPR. To identify regions of the DHPR important for receiving this signal from RyR-1, we expressed in dysgenic myotubes a chimera (SkLC) having skeletal (Sk) DHPR sequence except for a cardiac (C) II-III loop (L). Tagging with green fluorescent protein (GFP) enabled identification of expressing myotubes. Dysgenic myotubes expressing GFP-SkLC or SkLC lacked EC coupling and had very small Ca(2+) currents. Introducing a short skeletal segment (alpha(1S) residues 720-765) into the cardiac II-III loop (replacing alpha(1C) residues 851-896) of GFP-SkLC restored both EC coupling and Ca(2+) current densities like those of the wild type skeletal DHPR. This 46-amino acid stretch of skeletal sequence was recently shown to be capable of transferring strong, skeletal-type EC coupling to an otherwise cardiac DHPR (Nakai, J., Tanabe, T., Konno, T., Adams, B., and Beam, K.G. (1998) J. Biol. Chem. 273, 24983-24986). Thus, this segment of the skeletal II-III loop contains a motif required for both skeletal-type EC coupling and RyR-1-mediated enhancement of Ca(2+) current.
Grabner
M
M
Department of Anatomy and Neurobiology College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, Colorado 80523, USA.
Dirksen
R T
RT
Suda
N
N
Beam
K G
KG
D000595:Amino Acid Sequence; D000818:Animals; D000831:Animals, Newborn; D015220:Calcium Channels / Q000737:chemistry* / Q000235:genetics / Q000502:physiology*; D020746:Calcium Channels, L-Type; D002478:Cells, Cultured; D049452:Green Fluorescent Proteins; D006321:Heart / Q000502:physiology; D008164:Luminescent Proteins / Q000235:genetics; D008564:Membrane Potentials; D051379:Mice; D008817:Mice, Mutant Strains; D008969:Molecular Sequence Data; D018482:Muscle, Skeletal / Q000166:cytology / Q000502:physiology* / Q000503:physiopathology; D018408:Patch-Clamp Techniques; D017433:Protein Structure, Secondary; D020239:Receptor Cross-Talk / Q000502:physiology*; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000378:metabolism; D015183:Restriction Mapping; D019837:Ryanodine Receptor Calcium Release Channel / Q000235:genetics / Q000502:physiology*; D015398:Signal Transduction
D015220:Calcium Channels; D020746:Calcium Channels, L-Type; D008164:Luminescent Proteins; D011993:Recombinant Fusion Proteins; D019837:Ryanodine Receptor Calcium Release Channel; D049452:Green Fluorescent Proteins