, 1997), which may be necessary for survival.

Because these sterols are synthesized de novo by the organism despite its ability to scavenge available sterols, these Cyclopamine order sterols have been called ‘metabolic sterols’ (Haughan & Goad, 1991; Kaneshiro et al., 1994a), and because these sterols appear to be unique to Pneumocystis, they may not only provide excellent drug targets against the organism (Haughan & Goad, 1991), but they may have potential as possible markers for the detection of PCP (Kaneshiro et al., 1999). Cholesterol accounts for up to 81% of the total sterols isolated from Pneumocystis obtained from rat lungs, and it has been postulated that most, if not all, the cholesterol is scavenged from the host (Giner et al., 2002; Worsham et al., 2003). Conversely, one report speculates that P. carinii may synthesize cholesterol through a de novo pathway (Zhou et al., 2002), but to date, there is no evidence to suggest that the organism contains all of the genes necessary to synthesize either cholesterol or ergosterol. Despite the lack of detectable ergosterol in Pneumocystis membranes, genes involved in sterol synthesis have been identified within

its genome, and many of these genes have been R428 price proven functional based on targeted inhibition of these enzymes and the subsequent reduction in the viability of P. carinii (Kaneshiro et al., 2000). Figure 4 outlines the putative sterol biosynthetic pathway of P. carinii based on our current knowledge, and Table 1 lists Bcl-w P. carinii sterol enzymes and identifies the reaction products that have been detected in the membranes of the fungus. These

putative P. carinii sterol enzyme genes were identified based on sequence similarity to other known fungal sterol enzymes; however, functional analyses are necessary to determine their function. To date, only three of these genes, ERG7 (lanosterol synthase), ERG11 (lanosterol 14α demethylase) and ERG6 (sterol C-24 methyl transferase), have been the subject of research investigations. The activity of lanosterol synthase or Erg7 results in the conversion of the last acyclic sterol precursor into lanosterol, the first cyclic sterol intermediate of the sterol pathway. In Saccharomyces cerevisiae, loss of lanosterol synthase function results in a nonviable phenotype; similarly, inhibition of the P. carinii enzyme has been shown to reduce the viability of P. carinii in vitro (Kaneshiro et al., 2000). Saccharomyces cerevisiae Erg7 localizes to lipid particles, and when expressed in an S. cerevisiae ERG7 null mutant, homologs of Erg7 from the plant pathogen Arabidopsis thaliana and the parasite T. cruzi localized to lipid particles in an S. cerevisiae ERG7 mutant (Milla et al., 2002a, b). Lipid particles are thought to derive from the endoplasmic reticulum (ER), where neutral lipids accumulate within the lipid bilayer and bud off into the cytoplasm after reaching a certain size (Athenstaedt et al., 1999).