Fig. 1. Characterization of a novel protein forming mixed disulfides with Ero1-Lα. (A) Anti-myc immunoprecipitates from 107 HeLa cells stably transfected with a vector driving the expression of myc-tagged Ero1-Lα (Ero1-Lα) or with an empty vector (MOCK) were resolved by SDS–PAGE under non-reducing conditions, and stained with Coomassie Blue colloidal. Bands were excised and analysed by MS. The band denoted by an asterisk was shown to contain peptides derived from Ero1-Lα and from an open reading frame, corresponding to KIAA0573, subsequently resequenced from HeLa and SK-N-BE cDNAs. The faint band indicated by a dot was excised and was shown by MS to contain the same peptides as were found in the band indicated by the asterisk. (B) Sequence of the 44 kDa protein forming mixed disulfides with Ero1-Lα. The two peptides sequenced in the mixed disulfide with Ero1-Lα are underlined. Note the presence of a N-terminal hydrophobic sequence (shaded), which is absent in the mature protein, and an RDEL motif at the C-terminus (underlined). The six cysteine residues are indicated by dots. The trx-like domain is boxed. The CRFS motif has a black background. (C) Chromosomal organization of the gene encoding ERp44 in humans. Note the presence of an ERSE-like sequence upstream of the translation initiation site.