The subunit structure of Hsp80, the most abundant heat-shock protein of Neurospora crassa, was examined by chemical cross-linking of the purified protein in vitro. Resolution of glutaraldehyde-treated Hsp80 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis SDS-PAGE suggests that the native state of this protein is a tetramer; the relative proportion of cross-linked species, estimated by the fraction of protein recovered in each category, is consistent with a dimer-of-dimer structure. Upon interaction with nucleotides, higher order cross-linked oligomers were detected, indicating ligand-induced conformational changes. The effect of nucleotides was also monitored by following tryptophan fluorescence: CTP, UTP, and NAD led to fluorescence quenching, the effect of CTP being the most pronounced. As individual molecular chaperones often act in concert with cochaperones, interaction between the two major cytosolic stress proteins--Hsp80 and Hsp70--was examined. Purified Hsp70 was immobilized on ATP-agarose and purified Hsp80 was applied to the Hsp70-saturated matrix; while Hsp80 did not bind to ATP-agarose by itself, it was bound strongly by immobilized Hsp70. The [Hsp70-Hsp80] complex was eluted with ATP and coelution of both proteins was confirmed by Western blot analysis, using specific polyclonal antibodies raised against each protein. The physical association of stress-inducible Hsp70 and Hsp80 was verified by interprotein cross-linking in vitro followed by immunoblot analysis and by immunoprecipitation.

Heat shock protein 80 (Hsp80) of Neurospora crassa, a member of the stress-90 protein family, is a cytosolic molecular chaperone that interacts directly with Hsp70 to form a hetero-oligomeric complex. The complete nucleotide sequence of the gene encoding this protein, along with the 5'- and 3'-flanking DNA, is reported. The coding sequence is interrupted by two introns, 61 and 30 nucleotides, respectively, in length. The deduced amino acid sequence corresponds to a 695-residue polypeptide with a calculated molecular mass of 78,894 Da and an average pI of 4.94. Primer extension experiments demonstrated two transcription start sites, a major and a minor one. No sequence motifs resembling the standard eukaryotic heat shock elements were evident in the putative promoter region. Immunoblot analysis showed Hsp80 protein to be present in the mature, dormant conidia, while the hsp80 transcripts were not detected. Both the transcripts and the protein were present in the germinating conidia in the absence of externally applied stress.

A physical association between Hsp70 and Hsp80, the major cytosolic stress proteins of Neurospora crassa, was demonstrated previously by interprotein crosslinking and by binding of Hsp80 to Hsp70 immobilized on ATP-agarose. In the present study, [Hsp70: Hsp80] complex formation was analyzed by enzyme-linked immunosorbent assays (ELISA), using specific antibodies. One protein was fixed onto ELISA plate wells and binding of the second mobile protein was monitored by retention of its cognate IgG. Binding of Hsp70 and Hsp80 to immobilized Hsp80 and Hsp70, respectively, was readily detectable at submicrogram levels. The effect of cations and various nucleotides on [Hsp70: Hsp80] complex was examined by inclusion of KCl, MgCl2, MnCl2, and nucleotides in the interaction mixture. K+ stimulated interaction between immobilized Hsp70 and Hsp80 in solution and adenosine nucleotides exerted a stimulatory effect on complexation as well. Similarly, CTP, NAD, and NADH enhanced complex formation between plate-bound Hsp70 and Hsp80 in solution, attesting to the conformational flexibility of Hsp80. Epitope blocking revealed an overlap between protein-protein contact surfaces and antibody recognition sites. Binding to alpha-carboxymethylated lactalbumin showed that Hsp70 and Hsp80 can interact with an unfolded polypeptide, individually and in complex.

The process of repeat-induced point mutation (RIP) was used to disrupt hsps-1, the gene encoding the major heat-inducible member of the HSP70 family of Neurospora crassa. A plasmid DNA, containing an incomplete copy of hsps-1 and the selectable marker qa-2+, was introduced into germinated conidia. The sexual progeny of transformants with ectopically integrated hsps-1 DNA was examined for RIP by Southern-blot analysis of MboI- and Sau3A-digested genomic DNA. Progeny strains, showing RIP, were tested for heat shock-responsive expression of hsps-1, by RNA-blot hybridization and Western-blot analysis, as well as for thermotolerance. Isolates with RIP showed low levels of hsps-1 mRNA and a lack of induction of HSP70 protein by heat shock, accompanied by only a marginal decrease in the acquisition of thermotolerance.

Formation of a hetero-oligomeric complex between Hsp70 and Hsp80 of Neurospora crassa was observed previously by means of chemical crosslinking and enzyme-linked immunosorbent assays (ELISA). The present study documents the effect of nucleotides on the subunit structure of Hsp70 and Hsp80 by crosslinking with bifunctional reagents: glutaraldehyde, dimethyl adipimidate (DMA), and dimethyl suberimidate (DMS). The inter-protomer crosslinking of Hsp80 with DMA and DMS was suppressed by ATP and to a lesser extent by ADP, CTP, and NAD. Crosslinking of purified Hsp70 by glutaraldehyde yielded dimers and higher order oligomers. Binding of ATP, ADP, CTP, and NAD, but not NADH, led to a marked reduction in the yield of oligomers. Similarly, crosslinking by DMA and DMS was suppressed by ADP, ATP, and CTP. Both Hsp70 and Hsp80 exhibited intrinsic ATPase activity. Interestingly, ATP levels exceeding 25 microM resulted in pronounced inhibition of the ATPase activity of Hsp80 and 0.5 mM and 0.25 mM ATP led to a prolonged lag in the reaction. Addition of NAD resulted in the abolition of the lag period. The binding of 2-p-toluidinylnapthalene-6-sulfonate (TNS) to Hsp70 and its displacement by ATP and other nucleotides demonstrated the hydrophobic nature of the nucleotide-binding region.