The Hsp90 of Candida albicans can confer Hsp90 functions in Saccharomyces cerevisiae: a potential model for the processes that generate immunogenic fragments of this molecular chaperone in C. albicans infections

Abstract

During infections with a number of important eukaryotic pathogens the Hsp90 molecular chaperone of the pathogen is recognized as an immunodominant antigen by the host immune system. Yeast molecular genetics should allow study of the extent of sequence variation within conserved immunodominant epitopes on pathogen Hsp90s that is compatible with essential Hsp90 functions, as well as the processes that generate antigenic subfragments of these Hsp90s. The Hsp90 of the fungal pathogen Candida albicans was shown in this study to provide both essential and nonessential (pheromone signalling and mammalian steroid receptor activation) Hsp90 functions in Saccharomyces cerevisiae cells. Much of the C. albicans Hsp90 expressed in respiratory S. cerevisiae cells was shown to undergo a partial degradation in vivo, a degradation that closely resembles that of the native Hsp82 (one isoform of the homologous Hsp90) in S. cerevisiae. Allowing for the differences in the length of the charged linker region between the N-and C-terminal domains of C. albicans Hsp90 and S. cerevisiae Hsp82, these two proteins expressed in S. cerevisiae appear to give the same major degradation products. These Hsp90 fragments are similar to the products of incomplete Hsp90 degradation found in C. albicans cultures.

Expression of Genes Encoding Innate Host Defense Molecules in Normal Human Monocytes in Response toCandida albicans

“…HSPs may participate in innate immunity by inducing the secretion of inflammatory cytokines and chemokines (48). Although Candida HSPs have been studied (39,44), the role of human HSPs in host defense against candidiasis has not been investigated.…”

Preclinical Assessment of the Efficacy of Mycograb, a Human Recombinant Antibody against Fungal HSP90

“…This was usually associated with reactivity to bands at 40 and 92 kDa. The predicted molecular mass from the amino acid sequence of candidal HSP90 is 81 kDa (11), but when cloned into S. cerevisiae, it ran, on gel electrophoresis, at approximately 90 kDa, and both Mycograb and a mouse monoclonal against human HSP90 reacted with a band at 92 kDa (58,72,81). The pattern of cross-reactivity of rabbit and mouse monoclonal antibodies against specific HSP epitopes confirmed the relatedness of the three antigens: the parent molecule at 92 kDa was heat inducible while the 40-and 47-kDa proteins were expressed irrespective of whether the yeast was grown at 23 or 37°C or in the yeast or mycelial phase (44).…”

“…The importance of HSP90 to the viability of C. albicans is shown by the failure of repeated attempts to generate a homozygous C. albicans ⌬hsp90/⌬hsp90 null mutant (72). Total soluble protein extracts from exponentially growing C. albicans or Saccharomyces cerevisiae yield not only full-length HSP90 but also subfragments of 72 to 76 kDa and 47 kDa, which are the result of partial degradation within viable yeast cells (58). Examination of the antibody response in mice challenged with candidal protoplasts showed no evidence of an antibody response to HSP90 or its subfragments (61), indicating loss of this antigen family during removal of the yeast cell wall (75).…”

“…One of these antigens, at 48 kDa, was identified as enolase (34) and exploited as the basis of a diagnostic test (76). An antigen at 47 kDa has been identified as the carboxy fragment of C. albicans heat shock protein 90 (HSP90) (42,58,72). It is more abundant than, and clearly distinct from, enolase, as shown by the pattern of reactivity of an enolase-specific monoclonal antibody (18).…”

“…Homologous epitopes have been identified in a range of fungal species: Candida parapsilosis, Torulopsis glabrata, Candida tropicalis, Candida krusei, and Aspergillus fumigatus (9,12,26,38,44,66). HSP90 plays a critical role in chaperoning cellular molecules and is essential for yeast viability (46,58). The importance of HSP90 to the viability of C. albicans is shown by the failure of repeated attempts to generate a homozygous C. albicans ⌬hsp90/⌬hsp90 null mutant (72).…”