Functional analysis of putative phosphoenolpyruvate transporters localized to the Golgi apparatus in Schizosaccharomyces pombe

The cell surface of Schizosaccharomyces pombe is negatively charged due to the presence of pyruvylated oligosaccharides, which is important for cell-cell recognition. However, the mechanism of pyruvate supply to oligosaccharides is not clearly understood. Here, we analyzed three putative phosphoenolpyruvate (PEP) transporter genes (pet1⁺, pet2⁺, and pet3⁺) in S. pombe, identified by sequence homology search against the Arabidopsis thaliana PEP transporter AtPPT1. Schizosaccharomyces pombe strain carrying a disruption in pet1⁺ (pet1Δ) or in pet2⁺ (pet2Δ), but not the strain carrying a disruption in pet3⁺ (pet3Δ), showed reduced pyruvate level on the cell surface. This reduction in pyruvate level was restored to the control level by expressing green fluorescent protein (GFP)-tagged Pet1p and Pet2p in respective disruptants. Fluorescence microscope studies revealed that GFP-tagged Pet1p and Pet2p were localized to the Golgi apparatus. Although expression of neither AtPPT1 nor AtPPT2 suppressed the pet1Δ phenotype, that of chimeric constructs, where the N-terminal regions of AtPPT1 and AtPPT2 were replaced by the N-terminal region of Pet1p, partially suppressed the pet1Δ phenotype. Furthermore, the reduction in cell surface negative charge in pet1Δ cells was restored by incubating these cells with recombinant Pvg1p and PEP. Thus, Pet1p and Pet2p are likely involved in transporting PEP from the cytoplasm into the Golgi.