Calsequestrin gel conformation in situ

Calsequestrin (CASQ) is the major component of the sarcoplasmic reticulum (SR) lumen in skeletal and cardiac muscles. This calcium binding protein localizes to the junctional SR (jSR) cisternae where it is responsible for the storage of large amounts of Ca 2+ while it is usually absent, at least in its polymerized form, in the free SR. The retention of CASQ inside the jSR is partly due to its association with other jSR proteins like junctin and triadin and partly by its ability to polymerize, in a high Ca 2+ environment, into an intricate gel that holds the protein in place. In this work, we shed some light on the still poorly described in situ structure of polymerized CASQ using detailed electron microscopy images from thin sections, with and without tilting, and from deep etched rotary shadowed replicas. The latter directly illustrate the fundamental network nature of polymerized CASQ, revealing repeated nodal points connecting short segments of the linear