Re-evaluation of primary structure, topology, and localization of Scamper, a putative intracellular Ca2+ channel activated by sphingosylphosphocholine

Naturally occurring sphingoid molecules control vital functions of the cell through their interaction with specific receptors. Proliferation, differentiation and programmed death result in fact from a fine balance of signals, among which sphingosine and structurally related molecules play fundamental roles, acting as either first or second messengers. The corresponding receptors need to be identified in order that the role of sphingoid molecules can be established. Among them, several G-protein-coupled receptors specific for sphingosine 1-phosphate, sphingosylphosphocholine, or both, have already been investigated. In contrast, the identification of the postulated intracellular receptors has been problematical. In the present study we re-evaluated the molecular characterization of Scamper, the first proposed intracellular receptor for sphingosylphosphocholine [Mao, Kim, Almenoff, Rudner, Kearney and Kindman (1996) Proc. Natl. Acad. Sci. U.S.A. 93, 1993-1996] and commonly believed to be a Ca²⁺ channel of the endoplasmic reticulum (the name 'SCaMPER' used by Mao et al. being derived from 'sphingolipid Ca²⁺-release-mediating protein of the endoplasmic reticulum'). In contrast with what has been believed hitherto, our primary-structure and overexpression experiments indicate that Scamper is a 110-amino-acid protein spanning the membrane once with a Nexo/Ccyt topology [von Heijne and Gavel (1988) Eur. J. Biochem. 174, 671-678]. Overexpression of either wild-type or tagged Scamper induces a specific phenotype characterized by the rapid extension of actin-containing protrusions, followed by cell death.