Background: GPI anchor is essential for virulence of C. albicans. Little is known about its GPI biosynthetic pathway. We explore roles of two GPI-N-acetylglucosaminyltransferase subunits catalyzing the first step. Results: Subunits GPI2 and GPI19 are negatively co-regulated, affecting Ras1 activity and ERG11 levels, respectively. Conclusion: GPI2/GPI19 levels affect morphogenesis and ergosterol biosynthesis. Significance: C. albicans can be targeted by modulation of cross-talk among major pathways.

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Protein engineering is often applied to tailor substrate specificity, enantioselectivity, or stability of enzymes according to the needs of a process. In rational engineering approaches, molecular docking and molecular dynamics simulations are often used to compare transition states of wild-type and enzyme variants. Besides affecting the transition state energies by mutations, the entry of the substrate and its positioning in the active site (Michaelis complex) is also often studied, and mutagenesis of residues forming the substrate entry tunnel can have a profound impact on activity and selectivity. In this study, we combine the strengths of such a tunnel approach with MD followed by semiempirical QM calculations that allow the identification of beneficial positions and an in silico screening of possible variants. We exemplify this strategy in the expansion of the substrate scope of Chromobacterium violaceum amine transaminase toward sterically demanding substrates. Two double mutants (F88L/C418­(G/L)) proposed by the modeling showed >200-fold improved activities in the conversion of 1-phenylbutylamine and enabled the asymmetric synthesis of this amine from the corresponding ketone, which was not possible with the wild-type. The correlation of interaction energies and geometrical parameters (distance of the substrate’s carbonyl carbon to the cofactor’s amino group) as obtained in the simulations suggests that this strategy can be used for in silico prediction of variants facilitating an efficient entry and placement of a desired substrate as a first requirement for catalysis. However, when choosing amino acid positions for substitution and modeling, additional knowledge of the enzymatic reaction mechanism is required, as residues that are involved in the catalytic machinery or that guarantee the structural integrity of the enzyme will not be recognized by the developed algorithm and should be excluded manually.

Glycosylphosphatidyl inositol (GPI)-anchored proteins in Candida albicans are responsible for a vast range of functions, and deletions in certain GPI-anchored proteins severely reduce adhesion and virulence of this organism. In addition, completely modified GPIs are necessary for virulence. GPI anchor biosynthesis is essential for viability and starts with the transfer of N-acetylglucosamine to phosphatidylinositol. This step is catalysed by a multi-subunit complex, GPI–N-acetylglucosaminyltransferase (GPI–GnT). In this, the first report to our knowledge on a subunit of the Candida GPI–GnT complex, we show that CaGpi19p is the functional equivalent of the Saccharomyces cerevisiae Gpi19p. An N-terminal truncation mutant of CaGpi19p functionally complements a conditionally lethal S. cerevisiae gpi19 mutant. Further, we constructed a conditional null mutant of CaGPI19 by disrupting one allele and placing the remaining copy under the control of the MET3 promoter. Repression leads to growth defects, cell wall biogenesis aberrations, azole sensitivity and hyperfilamention. In addition, there is a noticeable gene dosage effect, with the heterozygote also displaying intermediate degrees of most phenotypes. The mutants also displayed a reduced susceptibility to the antifungal agent amphotericin B. Collectively, the results suggest that CaGPI19 is required for normal morphology and cell wall architecture.