FUNCTIONAL AND PHYSICAL INTERACTIONS BETWEEN PARTIAL MOLECULES OF STE6, A YEAST ATP-BINDING CASSETTE PROTEIN

The Saccharomyces cerevisiae a-factor transporter, STE6, is a member o f the ATP binding cassette (ABC) transporter superfamily, ABC proteins consist of four modular units that comprise two membrane spanning dom ains (MSDs) and two nucleotide-binding domains (NBDs). Like many ABC p roteins, STE6 contains these four domains in a single polypeptide; cer tain other ABC proteins are encoded as pairs of ''half-molecules'' or are further subdivided. Our previous studies demonstrated that STE6 ca n be expressed as two half-molecules that are functional when co-expre ssed, Here we dissect the interactions between modules of STE6 in grea ter detail, We show by co-immunoprecipitation that STE6 half-molecules interact physically, supporting the view that they co-assemble in viv o to form a functional transporter, We also demonstrate a physical int eraction between a STE6 half-molecule and full-length STE6; such compl exes appear to be functional, based on the striking finding that the d efective activity of full-length STE6 mutated in one of its NBDs can b e corrected by co-expression of the corresponding ''wild-type'' half-m olecule. We also show that a quarter-molecule consisting solely of the N-terminal MSD of STE6 can interact physically and functionally with a C-terminal three-quarter molecule of STE6, indicating that informati on directing the assembly of STE6 from partial molecules is contained, at least in part, within its membrane spans.