Analysis of Multiple Classes of Soybean Heat Shock Genes and Proteins

1996 
The influence of high temperature stress (heat shock or HS) and other environmental stress agents on gene expression of soybean seedlings has been extensively studied. The sequence analysis of HS genes has revealed a high degree of conservation among individual members of several heat shock protein (HSP) families and different classes within a family, but some interesting differences have been noted. These studies have also revealed complex patterns of regulation of expression of the HS genes and accumulation of the HSPs. Based primarily upon the deduced amino acid sequence of the HSPs, immunological cross-reactivity, and intracellular localization, the complex group of low molecular weight (LMW) HSP genes have been organized into multiple classes. In soybean several cDNA and genomic clones encoding 20 to 24 kD LMW HSPs have been isolated which represent new classes of the LMW HSP gene super family based on nucleotide/amino acid sequence and cell fractionation analyses. The mRNAs transcribed from these genes are of lower abundance than those for the 15 to 18 kD Class I and II proteins, and these genes occur as small multigene (i.e. three to four) classes or subfamilies. The mRNAs of three of these classes of LMW HSP genes are translated on ER- bound ribosomes and possess hydrophobic leader sequences. The presence of a consensus ER retention sequence on two of these proteins indicates that they probably reside within the ER. The third protein lacks the consensus ER retention signal and presumably is translocated to an as yet unidentified location. The mRNA representing a fourth LMW gene class is translated on unbound cytoplasmic ribosomes, and the predicted protein has a N-terminal sequence with properties similar to that of some proteins which are translocated into mitochondria. Early studies with soybean seedlings indicated that some 22 to 24 kD HSPs are localized in mitochondria. Differential induction by amino analog treatment indicates that genes assigned to the same class based on amino acid similarity and localization can be regulated differently. The possible role of the multiple classes on LMW 15 to 24 kD HSPs in protein protection from denaturation at high temperature (i.e. a chaperone function), based on studies from other laboratories is noted and some of these results will be summarized.
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