Comparative analysis of BiP gene expression in maize endosperm

Binding protein (BiP) is the endoplasmic reticulum member of the highly conserved HSP70 (heat shock protein 70) family of molecular chaperones. We have isolated and characterized two different BiP cDNA clones corresponding to genes expressed in immature kernels. These two cDNAs share extensive sequence similarity but map to unlinked loci in the maize genome. A comparison of the aa sequences predicted from the cDNA clones revealed only six aa differences between them. Investigation of gene-specific expression was carried out by RNA gel blot analysis. RNAs corresponding to both cDNA clones were present in increased amounts in the endosperm of floury-2 (fl2), Mucronate (Mc) and Defective endosperm-B30 (De*-B30) maize mutants, which produce abnormal storage proteins. Similar increases in RNAs corresponding to both probes were detected in cells treated with either of two agents that interfere with protein folding, azetidine-2-carboxylic acid (AZC) and tunicamycin. Investigation of the genomic complexity of the BiP genes by Southern blot analysis revealed several cross-hybridizing bands. These results are suggestive that the BiP genes expressed in endosperm are coordinately regulated members of a more complex maize BiP multigene family.