Structure and Function of Voltage-Dependent 
Calcium Channels 
Tsutomu Tanabe, Ph.D. — Assistant Investigator 
Dr. Tanabe is also Assistant Professor of Cellular and Molecular Physiology at Yale University School of 
Medicine. He received his B.A., M.A., and Ph.D. degrees from Kyoto University in Kyoto, Japan. He began 
studies on the structure and function of the receptors and ion channels in excitable membrane when he 
was a graduate student in Shosaku Numa's laboratory. After he received his doctorate, he continued to 
conduct research in Dr. Numa's laboratory as a faculty member before coming to Yale. 
VOLTAGE-dependent calcium channels play 
important roles in the regulation of a variety 
of cellular functions, including membrane excit- 
ability, muscle contraction, synaptic transmis- 
sion, and secretion. At least four types of calcium 
channels have been distinguished by their elec- 
trophysiological and pharmacological properties. 
Recently, molecular biological studies, com- 
bined with electrophysiology, have provided evi- 
dence that this diversity of calcium channels 
derives largely from differences in their pore- 
forming ttj-subunit and that the other subunits 
associated with can modify channel function. 
Furthermore, the diversity of the several subunits 
was shown to originate not only from differences 
in the genes encoding them but also from alterna- 
tive splicing of their RNAs. 
Several types of calcium channels are known to 
be coexpressed in single cells, and the cells ap- 
parently use them for different purposes. We are 
interested in the structure-function relationships 
of calcium channels and the molecular basis of 
one type's specialization. 
Muscular dysgenesis (mdg) is a fatal autosomal 
recessive mutation of mice. It eliminates excita- 
tion-contraction (E-C) coupling and dihydropyri- 
dine (DHP) -sensitive calcium current of the slow 
L-type from skeletal muscle. Analysis of genomic 
DNA and skeletal muscle RNA indicates that the 
mdg mutation is associated with alterations of the 
structural gene for the skeletal muscle DHP re- 
ceptor. Injection of an expression plasmid carry- 
ing the cDNA of the receptor restores both E-C 
coupling and skeletal L-type calcium current, 
suggesting a dual role of this molecule. 
The restored coupling resembles that of nor- 
mal skeletal muscle, which does not require 
entry of extracellular calcium ions. By contrast, 
injection into dysgenic myotubes of an expres- 
sion plasmid carrying cDNA of the cardiac DHP 
receptor produces rapid, cardiac-like L-type 
current and cardiac-type E-C coupling, which 
does require calcium ion entry. 
To investigate the molecular basis for these dif- 
ferences in calcium currents and E-C coupling, 
we expressed various cDNAs of the chimeric DHP 
receptor in dysgenic myotubes, with the follow- 
ing results. Expression of cDNAs encoding chi- 
meras with regions of the skeletal muscle DHP 
receptor replacing one or more of the corre- 
sponding large, putative cytoplasmic regions of 
the cardiac DHP receptor showed that the region 
linking repeats II and III is a major determinant of 
skeletal muscle-type E-C coupling. 
Expression of cDNAs encoding chimeras in 
which repeats of the skeletal muscle DHP recep- 
tor are replaced by corresponding repeats from 
the cardiac receptor showed that repeat I deter- 
mines whether the chimeric calcium channel ac- 
tivation will be slow (skeletal muscle-like) or 
rapid (cardiac-like). 
We are also focusing on the drug-binding sites of 
calcium channel molecules, including those of 
channel antagonist (L-type channel blocker), and 
the mechanism of calcium-dependent inactivation. 
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