The Arabidopsis thaliana RER1 gene family: its potential role in the endoplasmic reticulum localization of membrane proteins.

Many endoplasmic reticulum (ER) proteins are known to be localized to the ER by a mechanism called retrieval, which returns the molecules that are exported from the ER to the Golgi apparatus back to the ER. Signals are required to be recognized by this retrieval system. In the work on yeast Saccharomyces cerevisiae, we have demonstrated that transmembrane domains of a subset of ER membrane proteins including Sec12p, Sec71p and Sec63p contain novel ER retrieval signals. For the retrieval of these proteins, a Golgi membrane protein, Rer1p, is essential (Sato et al., Mol. Biol. Cell 6 (1995) 1459-1477; Proc. Natl. Acad. Sci. USA 94 (1997) 9693-9698). To address the role of Rer1p in higher eukaryotes, we searched for homologues of yeast from Arabidopsis thaliana. We identified three cDNAs encoding Arabidopsis counterparts of Rer1p with an amino acid sequence identity of 39-46% to yeast Rer1p and named and AtRer1Ap and AtRer1Bp are homologous to each other (85% identity), whereas AtRer1C1p is less similar to AtRer1Ap and AtRer1Bp (about 50%). Genomic DNA gel blot analysis indicates that there are several other genes, implying that Arabidopsis duanyu17951 constitutes a large gene family. The expression of these three genes is ubiquitous in various tissues but is significantly higher in roots, floral buds and a suspension culture in which secretory activity is probably high. All the three Atduanyu17951 cDNAs complement the yeast rer1 mutant and remedy the defect of Sec12p mislocalization. However, the degree of complementation differs among the three with that of being the lowest, again suggesting a divergent role of AtRer1C1p.

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