We have named this region the EXS family after (ERD1, XPR1, and SYG1). This family includes C-terminus portions from the SYG1 G-protein associated signal transduction protein from Saccharomyces cerevisiae, and sequences that are thought to be murine leukaemia virus (MLV) receptors (XPR1). N-terminus portions from these proteins are aligned in the SPX pfam03105 family. The previously noted similarity between SYG1 and MLV receptors over their whole sequences is thus borne out in pfam03105 and this family. While the N-termini aligned in pfam03105 are thought to be involved in signal transduction, the role of the C-terminus sequences aligned in this family is not known. This region of similarity contains several predicted transmembrane helices. This family also includes the ERD1 (ERD: ER retention defective) yeast proteins. ERD1 proteins are involved in the localization of endogenous endoplasmic reticulum (ER) proteins. erd1 null mutants secrete such proteins even though they possess the C-terminal HDEL ER lumen localization label sequence. In addition, null mutants also exhibit defects in the Golgi-dependent processing of several glycoproteins, which led to the suggestion that the sorting of luminal ER proteins actually occurs in the Golgi, with subsequent return of these proteins to the ER via `salvage' vesicles.
Jiyao W et al.(2023). "The conserved domain database in 2023.",