No abstract

Carbonic anhydrases (CAs) are metalloenzymes that catalyse the interconversion of carbon dioxide and bicarbonate with high efficiency. This reaction is fundamental to biological processes such as respiration, photosynthesis, pH homeostasis, CO2 transport and electrolyte secretion. CAs are distributed among all three domains of life, and are currently divided into five evolutionarily unrelated classes (α, β, γ, δ and ζ). Fungal CAs have only recently been identified and characterized in detail. While Saccharomyces cerevisiae and Candida albicans each have only one β-CA, multiple copies of β-CA- and α-CA-encoding genes are found in other fungi. Recent work demonstrates that CAs play an important role in the CO2-sensing system of fungal pathogens and in the regulation of sexual development. This review focuses on CA functions in S. cerevisiae, the fungal pathogens C. albicans and Cryptococcus neoformans, and the filamentous ascomycete Sordaria macrospora.

Carbon dioxide (CO2) is among the most important gases for all organisms. Its reversible interconversion to bicarbonate (HCO3 −) reaches equilibrium spontaneously, but slowly, and can be accelerated by a ubiquitous group of enzymes called carbonic anhydrases (CAs). These enzymes are grouped by their distinct structural features into α-, β-, γ-, δ- and ζ-classes. While physiological functions of mammalian, prokaryotic, plant and algal CAs have been extensively studied over the past years, the role of β-CAs in yeasts and the human pathogen Cryptococcus neoformans has been elucidated only recently, and the function of CAs in multicellular filamentous ascomycetes is mostly unknown. To assess the role of CAs in the development of filamentous ascomycetes, the function of three genes, cas1, cas2 and cas3 (carbonic anhydrase of Sordaria) encoding β-class carbonic anhydrases was characterized in the filamentous ascomycetous fungus Sordaria macrospora. Fluorescence microscopy was used to determine the localization of GFP- and DsRED-tagged CAs. While CAS1 and CAS3 are cytoplasmic enzymes, CAS2 is localized to the mitochondria. To assess the function of the three isoenzymes, we generated knock-out strains for all three cas genes (Δcas1, Δcas2, and Δcas3) as well as all combinations of double mutants. No effect on vegetative growth, fruiting-body and ascospore development was seen in the single mutant strains lacking cas1 or cas3, while single mutant Δcas2 was affected in vegetative growth, fruiting-body development and ascospore germination, and the double mutant strain Δcas1/2 was completely sterile. Defects caused by the lack of cas2 could be partially complemented by elevated CO2 levels or overexpression of cas1, cas3, or a non-mitochondrial cas2 variant. The results suggest that CAs are required for sexual reproduction in filamentous ascomycetes and that the multiplicity of isoforms results in redundancy of specific and non-specific functions.

The ubiquitous metalloenzyme carbonic anhy-