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

Abstract

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.

Dynamic Transcriptome Landscape of Maize Embryo and Endosperm Development

“…For example, Viviparous1 and LEC1 are important for embryogenesis and maturation (McCarty et al, 1991;Lotan et al, 1998). Other examples include O2, Shrunken2, Bt1, and Bt2, which are involved in storage compound accumulation (Russell et al, 1993;Vicente-Carbajosa et al, 1997;Shannon et al, 1998); Fie1 and Maternally expressed gene1, which are two endosperm imprinting genes (Gutiérrez-Marcos et al, 2004;Raissig et al, 2011); Glb1 and Glb2, which are two embryo storage protein genes (Kriz, 1989); and Floury2, which is a zein gene in the endosperm (Fontes et al, 1991). Such well-studied genes represent a small proportion of the 1,258 identified seed-specific genes, but this study provides information on when all of these genes are actively expressed (Supplemental Fig.…”

Enhanced Accumulation of BiP in Transgenic Plants Confers Tolerance to Water Stress

“…Plant BiP expression has also been shown to respond to a variety of abiotic and biotic stress conditions, such as water stress, fungus infestation, insect attack, nutritional stress, cold acclimation, and elicitors of the plantpathogenesis response (Anderson et al, 1994;Denecke et al, 1995;Kalinski et al, 1995;Fontes et al, 1996;Figueiredo et al, 1997;Fontes et al, 1999). In the endosperm of maize floury-2 mutant, the synthesis of a zein-like storage protein variant, which contains an uncleavable signal sequence, is associated with increased accumulation of BiP (Boston et al, 1991;Fontes et al, 1991;Coleman et al, 1995;Gillinkin et al, 1997). Likewise, the expression of an assembly-defective mutant of the bean storage protein phaseoline induces BiP synthesis in tobacco (Nicotiana tabacum L. cv Havana) leaf protoplasts (Pe-drazzini et al, 1994).…”

“…Likewise, the expression of an assembly-defective mutant of the bean storage protein phaseoline induces BiP synthesis in tobacco (Nicotiana tabacum L. cv Havana) leaf protoplasts (Pe-drazzini et al, 1994). Furthermore, tunicamycin, apotent activator of the UPR pathway, efficiently induces BiP expression at both mRNA and protein level in several plant systems (Fontes et al, 1991;D'amico et al, 1992;Figueiredo et al, 1997). These results have led to the conclusion that, like mammal and yeast (Saccharomyces cerevisiae) BiP, plant BiP is most likely regulated through a UPR pathway.…”

Gibberellic Acid Regulates the Level of a BiP Cognate in the Endoplasmic Reticulum of Barley Aleurone Cells

“…The deduced amino acid sequence of yeast BiP consists of 682 amino acids, including 58 aspartate and 53 glutamate residues that are implicated in calcium binding (14,16,17). A BiP homolog with an apparent mol wt of 70,000 has recently been identified in the endosperm of thefloury-2 mutant of corn (9). The deduced amino acid sequence ofthis BiP homolog shows it to be 62% identical to BiP from mouse and yeast (9).…”

“…A BiP homolog with an apparent mol wt of 70,000 has recently been identified in the endosperm of thefloury-2 mutant of corn (9). The deduced amino acid sequence ofthis BiP homolog shows it to be 62% identical to BiP from mouse and yeast (9).…”