Gene-gene interactions between mutants that accumulate abnormally high amounts of proglutelin in rice seed

Abstract

We had previously identified eight mutants, esp2 and g(G)lups1 to 7, which accumulated abnormally high amounts of proglutelin, the major storage protein in rice seeds. Analysis of their seed proteins by SDS-PAGE, their levels of the luminal chaperone BiP and gene-gene interactions indicated that these mutants fell into four classes. The most epistatic class consisted of esp2, which encodes a defective protein disulfide isomerase (PDI). A second class consisting of Glup1, glup2 and glup7 was hypostatic to esp2, and showed abnormally high levels of BiP, suggesting that maturation and export of proglutelins from the ER are inhibited in this class of mutants. The third class containing glup4, Glup5 and glup6 mutations was hypostatic to esp2, Glup1, glup2 and glup7. Since the glup4 allele encodes the small GTPase Rab5a, which participates in the trafficking of proglutelin from Golgi apparatus to the protein storage vacuole (PSV), this third class of mutants is likely affected in this process. Lastly, glup3, which encodes a vacuolar processing enzyme, which proteolytically processes proglutelin into acidic and basic subunits within the PSV, was hypostatic to the other mutants. Overall, these gene relationships are consistent with the sequential intracellular transport and processing of proglutelin and provide novel insights on the trafficking of proglutelin to the PSV.