Derlins belong to the rhomboid family. (a) ClustalW alignment (UniProtKB accession number indicated) of the transmembrane regions of Derlin-1 from Homo sapiens (hs, Q9BUN8), Danio rerio (dr, Q7ZVT9), Caenorhabditis elegans (ce, Q93561), Derlin-2 (hs, Q9GZP9) and Derlin-3 (hs, Q96Q80) from H. sapiens and GlpG from Shewanella trabarsenatis (st, Q0HDA1), GlpG, Yersinia frederiksenii (yf, C4SQG5), and Escherichia coli (ec, P09391). Pink, the active site dyad in GlpG, absent in Derlins. WR and GxxxG motifs are indicated by *. (b–d) Homology model of the Derlin-1 rhomboid domain. The positions of the WR motif (b), GxxxG motif (c) and positively charged residues on the cytoplasmic side of each transmembrane span in the homology model (d) are indicated. N- and C termini of Derlin-1 are located in the cytoplasm. (e) Representative immunofluorescence images of cells expressing N- and C-terminally tagged HA-Derlin-1-S in which the plasma membrane was permeabilized without (digitonin) or with (digitonin + Triton X-100) ER permeabilization. Scale bars are 10 μm. (f) Comparison of the four-pass topology model of Derlin-1 as inferred from the reported topology of the Saccharomyces cerevisiae ortholog Der1 (ref. ) with the six-pass topology model of Derlin-1 as predicted by homology with GlpG. (g) Derlin-1 spans the ER membrane six times. Immunoblot of lysates treated with or without Endo H from cells expressing Derlin-1-S containing a glycosylation acceptor sequence inserted after the indicated residue.