We sought evidence for precursors of the leukocyte integrin subunits αM and αX among unicellular eukaryotes such as Saccharomyces cerevisiae. Chromatography of cytosolic extracts of Saccharomyces cerevisiae on an affinity matrix coupled to BU-15, a monoclonal antibody that recognizes αX, revealed a band of Mr > 205 kDa under nonreducing conditions. Screening a λgt11 library of S. cerevisiae DNA with BU-15 (anti-αX) and anti-Mo1 (anti-αM) led to the isolation of a 3.7-kb EcoRI fragment containing the 3’ end of an open reading frame sufficient to encode a polypeptide in excess of 118 kDa. On the basis of Southern blotting at high stringency, this gene was present in S. cerevisiae, but not in other yeast species such as Candida glabrata. Analysis of the derived amino acid sequence demonstrated >98% identity with the S. cerevisiae protein Uso1p, a myosin-Iike polypeptide found exclusively in the cytosol. The C-terminal 1016 aa, expressed from the 3.7-kb EcoRI fragment in Escherichia coli as a β-galactosidase fusion protein, bound iC3b, a ligand for the I-domain in αM and αX, and were recognized by Mn41, a monoclonal antibody specific for the αM I-domain. Antigenic and functional conservation of an I-domain in S. cerevisiae suggests that this domain may be a prototype for integrin-like proteins in other primitive eukaryotes.