The mammalian BiP is regulated by phosphorylation, and it is generally accepted that its unmodified form constitutes the biologically active species. In fact, the glycosylation inhibitor tunicamycin induces dephosphorylation of mammalian BiP. The stress-induced phosphorylation state of plant BiP has not been examined. Here, we demonstrated that soybean BiP exists in interconvertible phosphorylated and nonphosphorylated forms, and the equilibrium can be shift to either direction in response to different stimuli. In contrast to tunicamycin treatment, water stress condition stimulated phosphorylation of BiP species in soybean cultured cells and stressed leaves. Despite their phosphorylation state, we demonstrated that BiP isoforms from water-stressed leaves exhibit protein binding activity, suggesting that plant BiP functional regulation may differ from other eukaryotic BiPs. We also compared the induction of the soybean BiP gene family, which consists of at least four members designated soyBiPA, soyBiPB, soyBiPC, and soyBiPD, by tunicamycin and osmotic stress. Although all soybean BiP genes were induced by tunicamycin, just the soyBiPA RNA was upregulated by osmotic stress. In addition, these stresses promoted BiP induction with different kinetics and acted synergistically to increase BiP accumulation. These results suggest that the soybean BiP gene family is differentially regulated by abiotic stresses through distinct signaling pathways.
The soybean binding protein (BiP) gene family consists of at least four members designated soyBiPA, soyBiPB, soyBiPC and soyBiPD. We have performed immunoblotting of two-dimensional (2D) gels and RT-PCR assays with gene-specific primers to analyze the differential expression of this gene family in various soybean organs. The 2D gel profiles of the BiP forms from different organs were distinct and suggested that the BiP genes are under organ-specific regulation. In fact, while all four BiP transcripts were detected in leaves by gene-specific reverse transcriptase-polymerase chain reaction (RT-PCR) assays, different subsets were detected in the other organs. The soyBiPD was expressed in all organs, whereas the expression of the soyBiPB was restricted to leaves. The soyBiPA transcripts were detected in leaves, roots and seeds and soyBiPC RNA was confined to leaves, seeds and pods. Our data are consistent with organ-specific expression of the soybean BiP gene family.
The binding protein BiP is an endoplasmic reticulum (ER)-resident member of the HSP70 stress-related protein family, which is essential for the constitutive function of the ER. In addition to responding to a variety of environmental stimuli, plant BiP exhibits a tissue-specific regulation. We have isolated two soybean BiP genomic clones, designated gsBiP6 and gsBiP9, and different extensions of their 5' flanking sequences were fused to beta-glucuronidase (GUS) reporter gene and introduced into Nicotiana tabacum by Agrobacterium tumefaciens-mediated transformation. Transgenic plants displayed prominent GUS activity in the vascular bundles of roots and shoots as well as in regions of intense cell division, such as procambial region and apical meristems. Promoter deletion analyses identified two cis-regulatory functional domains that are important for the spatially-regulated activation of BiP expression under normal plant development. While an AT-rich enhancer-like sequence, designated cis-acting regulatory domain 1, CRD1 (-358 to -211, on gsBiP6), activated expression of the BiP minimal promoter in all organs analyzed, BiP promoter activity in meristematic tissues and phloem cells required the presence of a second activating domain, CRD2 (-211 to -80). Apparently, the CRD2 sequence also harbors negative cis-acting elements, because removal of this region caused activation of gsBiP6 promoter in parenchymatic xylem rays. These results suggest that the tissue-specific control of BiP gene expression requires a complex integration of multiple cis-acting regulatory elements on the promoter.
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