In order to understand the biological complexity inherent to the pathogenicity of Candida albicans, gene expressions need to be analyzed at the protein level. We employed an epitope-tagging technique in a set of C albicans ORF, and constructed fifteen strains which expressed HA-tagged proteins. These efforts permitted us to identify differentially synthesized proteins during the hyphal differentiations. ICL1, MLS1, and WAP1, all of which are known to be hypha-induced at the transcript level, were indeed found to be up-regulated at the protein level. We also identified CaeIF4G, CaTPO5, and CaZRT1, the protein levels of which were increased during hyphal transition, and CaERB1, the protein level of which was reduced consistently. The hypha-induced protein level of CaeIF4G was closely associated with the cellular hyphal phenotype. CaeIF4G overexpression was shown to result in hyperfilamentation in C albicans. CaeIF4E, which was constitutively expressed during the hyphal development, exhibited no overexpression phenotype. HA-tagged strains were also utilized in our analysis of C albicans proteins in a co-culture of macrophage and C albicans. Five genes were found to be expressed differentially during the macrophage co-cultures. Our approaches proved to be rather useful under yeast culture conditions as well as in co-cultures of macrophage and C albicans. (c) 2005 Elsevier Inc. All rights reserved.