SUMMARY PARAGRAPH for PSA1
PSA1 encodes GDP-Man pyrophosphorylase (EC 220.127.116.11) (2, 4), which synthesizes GDP-mannose from mannose-1-phosphate and GTP. This is a necessary step in the preparation of mannose for incorporation into N-linked and O-linked glycoproteins, and psa1 mutants exhibit defects in both processes (2). In this pathway, Psa1p lies downstream of Sec53p and upstream of Dpm1p. Deletion of PSA1 is lethal (2, 5), but hypomorphic mutants exhibit a variety of phenotypes indicating a defect in cell wall biosynthesis. Specifically, psa1 mutants are sensitive to hypoosmolarity and require sorbitol in their growth medium (3); they are also unable to mate (3), hypersensitive to the drug geneticin (G418) (2), inefficiently transformed by electroporation or lithium acetate (3), and they leak cell surface proteins into the surrounding medium (3).
PSA1 was cloned as a multicopy suppressor (plasmid suppressor of alg1) that partially restored viability to an alg1 mutant strain in a cln1 cln2 background (5). PSA1 expression appears to be cell-cycle regulated, with transcription patterns similar to those of CLN2 and increasing 4-6x near the start of the cell cycle (5). PSA1 has been independently identified in several mutant screens and thus has multiple aliases, including VIG9 (vanadate-resistant and immature glycosylation) (2), SRB1 (requires sorbitol) (3, 6), and MPG1 (mannose-1-phosphate guanyltransferase) (4). The null phenotype is complemented by introduction of the Candida albicans (7, 8), Candida glabrata (8), or Kluyveromyces lactis (9) homologs.
Last updated: 2005-07-01