Virulence in Candida albicans correlates with adhesion to human epithelium. The C. albicans gene INT1 encodes a putative adhesin (Int1p) homologous to the mammalian integrins αM/αX. The nonpathogenic, nonadhesive, nonfilamentous yeast S. cerevisiae does not contain INT1. When INT1 is expressed in S. cerevisiae, germ tube formation and adhesion to human epithelium parallels that of C. albicans. Because there are multiple mediators of candidal adhesion to epithelium, we sought to map the adhesive domains of Int1p in nonadherent yeast S. cerevisiae. Sequence analysis of INT1 predicts a 188 kD polypeptide including several motifs common to αM/αX [an I domain, two divalent cation binding sites (CBS1& CBS2), a transmembrane domain (TM) and a cytoplasmic tail with a single tyrosine residue (CYTO)], as well as an internal RGD site not found in integrins. INT1 constructs involving deletions/substitutions of these sites were subcloned into a yeast shuttle vector (pBM272) and expressed in S. cerevisiae YPH500 under the control of a galactose-inducible promoter. All transformed yeast strains showed normal growth curves in standard and selective media. All mutants, save the RGD deletion, formed INT1-dependent germ tubes when grown in galactose. Western blot analysis using Int1p specific antibodies revealed a protein of 200 ± 20 kD in all transformants but the RGD mutant. Epithelial adhesion of S. cerevisiae expressing INT1 and each mutant gene was determined. Adhesion of the yeast expressing the CBS2 construct was reduced but not significantly different from that expressing native INT1(24.9±4.4 vs 32.6±1.4). Specific adhesion was significantly reduced for S. cerevisiae expressing the TMCYTO (14.9 ± 2.4) and RGD (12.5 ± 2.5) constructs (p<0.001). Adhesion of the S. cerevisiae transformed with vector alone was 4.9 ± 1.2. These results provide 1) confirmation of INT1-mediated adhesion in S. cerevisiae; 2) further evidence that INT1-mediated adhesion is not calcium dependent; 3) a novel role for the TMCYTO domain in epithelial adhesion.