Subunit structure of junctional feet in triads of skeletal muscle: a freeze-drying, rotary-shadowing study.

Abstract

Isolated heavy sarcoplasmic reticulum vesicles retain junctional specializations (feet) on their outer surface. We have obtained en face three-dimensional views of the feet by shadowing and replicating the surfaces of freeze-dried isolated vesicles. Feet are clearly visible as large structures located on raised platforms. New details of foot structure include a four subunit structure and the fact that adjacent feet do not abut directly corner to corner but are offset by half a subunit. Feet aligned within rows were observed to be rotated at a slight angle off the long axis of the row creating a center-to-center spacing (32.5 nm) slightly less than the average diagonal of the feet (35.3 nm). Comparison with previous information from thin sections and freeze-fracture showed that this approach to the study of membranes faithfully preserves structure and allows better visualization of surface details than either thinsectioning or negative-staining.

Type 3 ryanodine receptors of skeletal muscle are segregated in a parajunctional position

“…Two rows of junctional feet (blue profiles) occupy the long axis of the junction. The feet interact with each other close to their corners, and their position is slightly skewed relative to the long axis of the junction (27). Tetrads in the T tubules are associated with alternate junctional feet.…”

Locations of Calmodulin and FK506-binding Protein on the Three-dimensional Architecture of the Skeletal Muscle Ryanodine Receptor

“…RyRs in vertebrate skeletal muscle interact to form ordered arrays consisting of two or three rows of receptors in which adjacent RyRs interact with each other isologously near their corners, but with some overlap (Ϸ12 nm) of their edges (14,42,60). Electron microscopy studies indicate that not all of the RyRs are associated with voltage sensors (23,59), and it has been postulated that the gating of these non-DHPR-liganded RyRs is controlled by interacting with adjacent RyRs that are associated with voltage sensors.…”

Isolation and partial cloning of ryanodine‐sensitive Ca2+ release channel protein isoforms from human myometrial smooth muscle

“…The skeletal (RyR1) and cardiac (RyR2) channels comprise 30S homotetrameric complexes of ~ 565 kDa subunits that form a 'four leaf clover-like' structure and display a morphology identical to the 'feet' structures that span the T tubule/SR junctional gap [11]. RyR1 is found predominantly in skeletal muscle and has also been identified in oesophagus and regions throughout the brain [12].…”