Mass spectrometric quantification of the adaptations in the wall proteome of Candida albicans in response to ambient pH

Abstract

The mucosal layers colonized by the pathogenic fungus Candida albicans differ widely in ambient pH. Because the properties and functions of wall proteins are probably pH dependent, we hypothesized that C. albicans adapts its wall proteome to the external pH. We developed an in vitro system that mimics colonization of mucosal surfaces by growing biomats at pH 7 and 4 on semi-solid agarose containing mucin as the sole nitrogen source. The biomats expanded radially for at least 8 days at a rate of ∼30 μm h−1. At pH 7, hyphal growth predominated and growth was invasive, whereas at pH 4 only yeast and pseudohyphal cells were present and growth was noninvasive. Both qualitative mass spectrometric analysis of the wall proteome by tandem mass spectrometry and relative quantification of individual wall proteins (pH 7/pH 4), using Fourier transform mass spectrometry (FT-MS) and a reference mixture of 15N-labelled yeast and hyphal walls, identified similar sets of >20 covalently linked wall proteins. The adhesion proteins Als1 and Als3, Hyr1, the transglucosidase Phr1, the detoxification enzyme Sod5 and the mammalian transglutaminase substrate Hwp1 (immunological detection) were only present at pH 7, whereas at pH 4 the level of the transglucosidase Phr2 was >35-fold higher than at pH 7. Sixteen out of the 22 proteins identified by FT-MS showed a greater than twofold change. These results demonstrate that ambient pH strongly affects the wall proteome of C. albicans, show that our quantitative approach can give detailed insights into the dynamics of the wall proteome, and point to potential vaccine targets.