Stromal interaction molecule 1 (STIM1), a transmembrane protein with growth suppressor activity, contains an extracellular SAM domain modified by N-linked glycosylation

Stromal interaction molecule I (STIMI) is a cell surface transmembrane glycoprotein implicated in tumour growth control and stromal-haematopoictic cell interactions. A single sterile alpha motif (SAM) protein-protein interaction domain is modelled within its extracellular region, a subcellular localisation not previously described for other SAM domain-containing proteins. We have defined the transmembrane topology of STIMI by determining the sites of N-linked glycosylation. We have confirmed that STIMI is modified by N-linked glycosylation at two sites within the SAM domain itself, deduced as asparagine residues N131 and N171, demonstrating that STIMI is translocated across the membrane of the endoplasmic reticulum such that the SAM domain resides within the endoplasmic reticulum (ER) lumen. Both N-linked oligosaccharides remain endoglycosidase H-sensitive, indicating absence of full processing within the ER and Golgi. This immature modification is nevertheless sufficient and critical for cell surface expression of STIM1 We show that STIM1-STIM1 homotypic interactions are mediated via the cytoplasmic rather than the extracellular region of STIMI, excluding an essential role for the SAM domain in these protein interactions. These studies provide the first evidence for an extracellular localisation of a SAM domain within any protein, and the first example of a SAM domain modified by N-linked glycosylation. (C) 2002 Elsevier Science B.V. All rights reserved.