The yeast Saccharomyces cerevisiae ste6 mutant is defective in transport of a-mating factor, resulting in an inability of ste6 a cells to mate with alpha cells. The gene encodes an ATP-binding cassette, ABC transporter. We used functional complementation of a yeast ste6 mutant with an Arabidopsis thaliana expression library in an attempt to clone an Arabidopsis homolog. Sequence analysis of the isolated Arabidopsis complementing cDNA however showed no homology to the STE6 gene. High sequence similarity was detected to members of the mitogen-activated serine/threonine protein (MAP) kinase family involved in signal transduction: STE20, STE11, BCK1, Byr2 and p65PAK. The Arabidopsis clone failed to complement a fus3/kss1 mutant of S. cerevisiae, but did complement a defect in ste11, ste20, as well as ste6. The isolated clone encodes a protein that is truncated at its amino-terminal, and might function in a similar way as a dominant STE11 truncation allele. These results suggest that the Arabidopsis cDNA encodes a putative serine/threonine kinase that can function in the mating response pathway upstream of FUS3/KSS1 in S. cerevisiae, at the level of STE11 gene. Interestingly, this clone is able to restore the ability of the ste6 yeast mutant to export a-factor.