SID1 transmembrane family, member 2 (Sidt2): a novel lysosomal membrane protein.

In a recent proteomic study of lysosomal proteins [10], we identified SID1 transmembrane family, member 2 (Sidt2) as a novel lysosomal membrane protein candidate. The Sidt2 gene encodes an 832-amino acid residues protein with a calculated molecular mass of 94.5kDa. Bioinformatic analysis showed that Sidt2 is a multipass transmembrane protein that contains 10 putative N-glycosylation sites (NxS/T) and two potential tyrosine-based sorting signals (YGSF and YDTL). Using specific anti-Sidt2 antibody and lysosomal markers, the lysosomal localization of Sidt2 was determined by immunofluorescence. Furthermore, using subcellular fractionation techniques, we demonstrated that Sidt2 is a lysosomal integral membrane protein. Endogenous Sidt2 was detected in multiple tissues of mouse and rat with approximately 120-130kDa molecular weights due to extensive glycosylation. After digestion with PNGase F, the apparent molecular mass of Sidt2 decreased to the predicted value of 95kDa. In rats, Sidt2 was highly expressed in the liver, brain, and kidney, whereas no or little expression was found in the skeletal muscles, heart, and other tissues. In summary, Sidt2 is a highly glycosylated lysosomal integral membrane protein that shows tissue-specific expression.