SRP-27 (sarcoplasmic reticulum protein of 27 kDa) is a newly identified integral membrane protein constituent of the skeletal muscle SR (sarcoplasmic reticulum). We identified its primary structure from cDNA clones isolated from a mouse skeletal muscle cDNA library. ESTs (expressed sequence tags) of SRP-27 were found mainly in cDNA libraries from excitable tissues of mouse. Western blot analysis confirmed the expression of SRP-27 in skeletal muscle and, to a lower extent, in heart and brain. Mild trypsin proteolysis combined with primary-structure prediction analysis suggested that SRP-27 has four transmembrane-spanning alpha helices and its C-terminal domain faces the cytoplasmic side of the endo(sarco)plasmic reticulum. The expression of SRP-27 is higher in fast twitch skeletal muscles compared to slow twitch muscles and peaks during the first month of post-natal development. High-resolution immunohistochemistry and Western blot analysis of subcellular fractions indicated that SRP-27 is distributed in both longitudinal tubules and terminal cisternae of the SR, as well as in the perinuclear membrane systems and the nuclear envelope of myotubes and adult fibres. SRP-27 co-sediments with the RyR (ryanodine receptor) macromolecular complex in high-salt sucrose-gradient centrifugation, and is pulled-down by anti-RyR as well as by maurocalcin, a well characterized RyR modulator. Our results indicate that SRP-27 is part of a SR supramolecular complex, suggesting the involvement of SRP-27 in the structural organization or function of the molecular machinery underlying excitation–contraction coupling.