Characterization of an lmmunoglobulin Binding Protein Homolog in the Maize floury-2 Endosperm Mutant

The maize b-70 protein is an endoplasmic reticulum protein overproduced in the floury-2 (f12) endosperm mutant. The increase in b-70 levels in f12 plants occurs during seed maturation and is endosperm specific. We have used amino acid sequence homology to identify b-70 as a homolog of mammalian immunoglobulin binding protein (BiP). Purified b-70 fractions contain two 75-kilodalton polypeptides with pl values of 5.3 and 5.4. Both 75-kilodalton polypeptides share severa1 properties with BiP, including the ability to bind ATP and localization within the lumen of the endoplasmic reticulum. In addition, both b-70 polypeptides can be induced in maize cell cultures with tunicamycin treatment. Like BiP, the pl5.3 form of b-70 is post-translationally modified by phosphorylation and ADPribosylation. However, modification of the pl 5.4 species was not detected in vitro or in vivo. Although the b-70 gene is unlinked to f12, b-70 overproduction is positively correlated with the f12 gene and is regulated at the mRNA level. In contrast, the f12 allele negatively affects the accumulation of the major endosperm storage proteins. The physical similarity of b-70 to BiP and its association with abnormal protein accumulation in f12 endoplasmic reticulum may reflect a biological function to mediate protein folding and assembly in maize endosperm.