Characterization of an immunoglobulin 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 (fl2) endosperm mutant. The increase in b-70 levels in fl2 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 several 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 pl 5.3 form of b-70 is post-translationally modified by phosphorylation and ADP-ribosylation. However, modification of the pl 5.4 species was not detected in vitro or in vivo. Although the b-70 gene is unlinked to fl2, b-70 overproduction is positively correlated with the fl2 gene and is regulated at the mRNA level. In contrast, the fl2 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 fl2 endoplasmic reticulum may reflect a biological function to mediate protein folding and assembly in maize endosperm.
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Department of Botany, North Carolina State University, Raleigh 27695-7612.
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D000244:Adenosine Diphosphate / Q000378:metabolism; D000595:Amino Acid Sequence; D000818:Animals; D029681:Arabidopsis Proteins; D001483:Base Sequence; D002352:Carrier Proteins / Q000235:genetics* / Q000378:metabolism; D002478:Cells, Cultured; D002851:Chromatography, High Pressure Liquid; D004247:DNA; D015180:Electrophoresis, Gel, Two-Dimensional; D000091342:Endoplasmic Reticulum Chaperone BiP; D006360:Heat-Shock Proteins / Q000235:genetics* / Q000378:metabolism; D018832:Molecular Chaperones; D008969:Molecular Sequence Data; D009154:Mutation; D010766:Phosphorylation; D010940:Plant Proteins / Q000235:genetics* / Q000378:metabolism; D012639:Seeds / Q000378:metabolism*; D017386:Sequence Homology, Amino Acid; D014415:Tunicamycin / Q000494:pharmacology; D003313:Zea mays / Q000235:genetics*
D029681:Arabidopsis Proteins; D002352:Carrier Proteins; D000091342:Endoplasmic Reticulum Chaperone BiP; D006360:Heat-Shock Proteins; D018832:Molecular Chaperones; D010940:Plant Proteins; D014415:Tunicamycin; C077221:BIP protein, Arabidopsis; D000244:Adenosine Diphosphate; D004247:DNA