MOLECULAR CHARACTERIZATION OF CDNAS ENCODING LOW-MOLECULAR-WEIGHT HEAT-SHOCK PROTEINS OF SOYBEAN

Three cDNA clones (GmHSP23.9, GmHSP22.3, and GmHSP22.5) representing t hree different members of the low-molecular-weight (LMW) heat shock pr otein (HSP) gene superfamily were isolated and characterized. A fourth cDNA clone, pFS2033, was partially characterized previously as a full -length genomic clone GmHSP22.0. The deduced amino acid sequences of a ll four cDNA clones have the conserved carboxyl-terminal LMW HSP domai n. Sequence and hydropathy analyses of GmHSP22, GmHSP22.3, and GmHSP22 .5, representing HSPs in the 20 to 24 kDa range, indicate they contain amino-terminal signal peptides. The mRNAs from GmHSP22, GmHSP22.3, an d GmHSP22.5 were preferentially associated in vivo with endoplasmic re ticulum (ER)-bound polysomes. GmHSP22 and GmHSP22.5 encode strikingly similar proteins; they are 78% identical and 90% conserved at the amin o acid sequence level, and both possess the C-terminal tetrapeptide KQ EL which is similar to the consensus ER retention motif KDEL; the enco ded polypeptides can be clearly resolved from each other by two-dimens ional gel analysis of their hybrid-arrest translation products. GmHSP2 2.3 is less closely related to GmHSP22 (48% identical and 70% conserve d) and GmHSP22.5 (47% identical and 65% conserved). The fourth cDNA cl one, GmHSP23.9, encodes a HSP of ca. 24 kDa with an amino terminus tha t has characteristics of some mitochondrial transit sequences, and in contrast to GmHSP22, GmHSP22.3, and GmHSP22.5, the corresponding mRNA is preferentially associated in vivo with free polysomes. It is propos ed that the LMW HSP gene superfamily be expanded to at least six class es to include a mitochondrial class and an additional endomembrane cla ss of LMW HSPs.