title: The Saccharomyces cerevisiae LEP1/SAC3 gene is associated with leucine transport.
abstract: Leucine uptake by Saccharomyces cerevisiae is mediated by three transport systems, the general amino acid transport system (GAP), encoded by GAP1, and two group-specific systems (S1 and S2), which also transport isoleucine and valine. A new mutant defective in both group-specific transport activities was isolated by employing a gap1 leu4 strain and selecting for trifluoroleucine-resistant mutants which also showed greatly reduced ability to utilize L-leucine as sole nitrogen source and very low levels of [14C]L-leucine uptake. A multicopy plasmid containing a DNA fragment which complemented the leucine transport defect was isolated by selecting for transformants that grew normally on minimal medium containing leucine as nitrogen source and subsequently assaying [14C]L-leucine uptake. Transformation of one such mutant, lep1, restored sensitivity to trifluoroleucine. The complementing gene, designated LEP1, was subcloned and sequenced. The LEP1 ORF encodes a large protein that lacks characteristics of a transporter or permease (i.e., lacks hydrophobic domains necessary for membrane association). Instead, Lep1p is a very basic protein (pI of 9.2) that contains a putative bipartite signal sequence for targeting to the nucleus, suggesting that it might be a DNA-binding protein. A database search revealed that LEP1 encodes a polypeptide that is identical to Sac3p except for an N-terminal truncation. The original identification of SAC3 was based on the isolation of a mutant allele, sac3-1, that suppresses the temperature-sensitive growth defect of an actin mutant containing the allele act1-1. Sac3p has been previously shown to be localized in the nucleus. When a lep1 mutant was crossed with a sac3 deletion mutant, no complementation was observed, indicating that the two mutations are functionally allelic.
authors.affiliation: Departamento de Bioquimica, Facultad de Medicina, Universidad de Buenos Aires, Argentina. cstella@mail.fmed.uba.ar
authors.forename: C A
authors.initials: CA
authors.lastname: Stella
authors.forename: C
authors.initials: C
authors.lastname: Korch
authors.forename: E H
authors.initials: EH
authors.lastname: Ramos
authors.forename: A
authors.initials: A
authors.lastname: Bauer
authors.forename: R
authors.initials: R
authors.lastname: Kölling
authors.forename: J R
authors.initials: JR
authors.lastname: Mattoon
mesh_terms: D000483:Alleles; D026905:Amino Acid Transport Systems; D001483:Base Sequence; D001692:Biological Transport; D003001:Cloning, Molecular; D004271:DNA, Fungal; D004352:Drug Resistance, Microbial; D005656:Fungal Proteins / Q000235:genetics / Q000378:metabolism*; D005800:Genes, Fungal; D005816:Genetic Complementation Test; D007930:Leucine / Q000031:analogs & derivatives / Q000378:metabolism* / Q000494:pharmacology; D026901:Membrane Transport Proteins; D008969:Molecular Sequence Data; D009154:Mutation; D009687:Nuclear Proteins / Q000235:genetics / Q000378:metabolism*; D029543:Nucleocytoplasmic Transport Proteins; D018272:Porins; D012441:Saccharomyces cerevisiae / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins
chemical_list: D026905:Amino Acid Transport Systems; D004271:DNA, Fungal; D005656:Fungal Proteins; D026901:Membrane Transport Proteins; D009687:Nuclear Proteins; D029543:Nucleocytoplasmic Transport Proteins; D018272:Porins; C103382:SAC3 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; C035177:5',5',5'-trifluoroleucine; D007930:Leucine