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* Department of Cell Developmental Biology, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6058; and Rapid release of calcium from the sarcoplasmic reticulum (SR) of skeletal muscle fibers during excitation-contraction (e-c) coupling is initiated by the
interaction of surface membrane calcium channels (dihydropyridine receptors; DHPRs) with the calcium release channels of the SR (ryanodine receptors; RyRs,
or feet). We studied the early differentiation of calcium
release units, which mediate this interaction, in BC3H1
cells. Immunofluorescence labelings of differentiating
myocytes with antibodies against
Department of Biochemical Pharmacology, University of Innsbruck, A-6020 Innsbruck, Austria
1 and
2 subunits of
DHPRs, RyRs, and triadin show that the skeletal isoforms of all four proteins are abundantly expressed
upon differentiation, they appear concomitantly, and
they are colocalized. The transverse tubular system is
poorly organized, and thus clusters of e-c coupling proteins are predominantly located at the cell periphery.
Freeze fracture analysis of the surface membrane reveals tetrads of large intramembrane particles, arranged in orderly arrays. These appear concomitantly
with arrays of feet (RyRs) and with the appearance of
DHPR/RyS clusters, confirming that the four components of the tetrads correspond to skeletal muscle
DHPRs. The arrangement of tetrads and feet in developing junctions indicates that incorporation of DHPRs
in junctional domains of the surface membrane proceeds gradually and is highly coordinated with the formation of RyR arrays. Within the arrays, tetrads are
positioned at a spacing of twice the distance between
the feet. The incorporation of individual DHPRs into
tetrads occurs exclusively at positions corresponding to
alternate feet, suggesting that the assembly of RyR arrays not only guides the assembly of tetrads but also determines their characteristic spacing in the junction.
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