Jcm. Cascardo et al., The phosphorylation state and expression of soybean BiP isoforms are differentially regulated following abiotic stresses, J BIOL CHEM, 275(19), 2000, pp. 14494-14500
The mammalian BiP is regulated by phosphorylation, and it is generally acce
pted 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 interc
onvertible phosphorylated and nonphosphorylated forms, and the equilibrium
can be shift to either direction in response to different stimuli. In contr
ast to tunicamycin treatment, water stress condition stimulated phosphoryla
tion of BiP species in soybean cultured cells and stressed leaves. Despite
their phosphorylation state, we demonstrated that BiP isoforms from water-s
tressed leaves exhibit protein binding activity, suggesting that plant BiP
functional regulation may differ from other eukaryotic BiPs, We also compar
ed the induction of the soybean BiP gene family, which consists of at least
four members designated soyBiPA, soyBiPB, soyBiPC, and soyBiPD, by tunicam
ycin and osmotic stress. Although all soybean BiP genes were induced by tun
icamycin, just the soyBiPA RNA was upregulated by osmotic stress. In additi
on, these stresses promoted BiP induction with different kinetics and acted
synergistically to increase BiP accumulation. These results suggest that t
he soybean BiP gene family is differentially regulated by abiotic stresses
through distinct signaling pathways.