Abstract
Molecular chaperones are important in folding newly synthesized polypeptides, assembly of multi-subunit structure and stabilization of proteins unfolded during cellular stresses. Many chaperones were discovered as heat shock proteins (Hsps) from prokaryotes to eukaryotes and classified into groups by the different molecular weight. Hsp70s/Hsc70s bind theirs substrates through hydrophobic interactions and were encoded by gene families in the plastid, mitochondrion, cytoplasm and endoplastic reticulum. The expression of the heat-shock genes is regulated by cis-regulatory promoter elements (HSEs) and trans-active heat shock transcription factors (HSFs). Hsp70s/Hsc70s consist of a highly conserved N-terminal 44-kDa ATPase domain, a less conserved 18-kDa peptide-binding domain and a C-terminal 10-kDa variable domain. DnaJ/Hsp40 stimulates the ATPase activity by accelerating the rate of ATP hydrolysis. Hop/p60 provides a physical link between Hsp70 and Hsp90. These two cofactors bind to the 10-kDa C-terminal domain of Hsc70 in a noncompetitive manner. Bag-1/GrpE is the nucleotide release factor stimulating exchange of ADP for ATP. On the other hand, Hip/p48 stabilizes the ADP bound form by inhibiting the release of nucleotide and has been shown to compete the function of Bag-1 in binding to the ATPase domain.Two bands of tomato Hsc70s were detected in seeds, but only one band was detected in leaves. Expression of different Hsc70s at different developmental stages or under stress may be related to the Hsc70s functions and regulation. PET chimeras were constructed to obtain different subdomains of tomato Hsc70s for studying differences of substrate binding in plants and animals. We also performed phage-display selections to screen the peptides bound to Lehsc70-1 30-kDa protein. The observed frequency of HPMSRPR was 37.5% in the 16 selected peptides. A animal Hsc70s contain more hydrophobic residues and plant Hsc70s contain more hydrophilic residues in 10-kDa domains of Hsc70.