Fig 1: Phosphorylated glycogen phosphorylase, GPa, localizes at SR.(A) Sections of a z-stack (corrected as in Figure 4) of human muscle co-stained for GPa and Casq1(I.D. #164, MHN). (B) 3-D renderings of the z-stacks that contained the sections in A. Both proteins delineate two terminal cisternae, TC, in every triad. The inset in Ac shows close proximity of the imaged proteins, without actual colocalization (documented with Figure 5—figure supplement 1). (C, D) Sections and stack from a human myofiber co-stained for GPa and STIM1 (I.D. #164, MHN). GPa delineates two TC in every triad (Ca, Da). STIM1 forms clusters in close proximity to the GPa bands, without actual colocalization. Minor quantities of both proteins are located outside triads. A large quantity of STIM1, unaccompanied by GPa, appears inside a nucleus (cf. (Lee et al., 2018). These images are the first to resolve TC with conventional optics (a resolution of ~100 nm is estimated with Figure 5—figure supplement 2).