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Structural Organization of the Spinach Endoplasmic Reticulum-Luminal 70-Kilodalton Heat-Shock Cognate Gene and Expression of 70-Kilodalton Heat-Shock Genes during Cold Acclimation

The 70-kD heat-shock proteins (HSP70s) are encoded by a multigene family in eukaryotes. In plants, the 70-kD heat-shock cognate (HSC70) proteins are located in organellar and cytosolic compartments of cells in most tissues. Previous work has indicated that HSC70 proteins of spinach (Spinacia oleracea) are actively synthesized during cold-acclimating conditions. We have isolated, sequenced, and characterized cDNA and genomic clones for the endoplasmic reticulum (ER) luminal HSC70 protein (immunoglobulin heavy chain-binding protein; BiP) of spinach. The spinach ER-luminal HSC70 is a constitutively expressed gene consisting of eight exons. Spinach BiP mRNA appears to be up-regulated during cold acclimation but is not expressed during water stress or heat shock. In contrast to the differential regulation of mRNA, the ER-luminal HSC70 protein levels remain constant in response to various environmental stresses. Two other members of the spinach 70-kD heat-shock (HS70) multigene family also show differential expression in response to a variety of environmental stresses. A constitutively expressed cytosolic HSC70 protein in spinach appears also to be up-regulated in response to both cold-acclimating and heat-shock treatments. Spinach also contains a cold-shock-induced HS70 gene that is not expressed during heat shock or water stress. Since HSP70s are considered to be involved with the chaperoning and folding of proteins, the data further support the concept that they may be important for maintaining cellular homeostasis and proper protein biogenesis during cold acclimation of spinach.