Functional Expression and Cellular Localization of a Green Fluorescent Protein-Tagged Proline Transporter in Aspergillus nidulans

Abstract

The PrnB protein is a highly specific proline transporter that belongs to an amino acid transporter family conserved in both prokaryotes and eukaryotes. In this work, we detected and analyzed the cellular localization of PrnB in vivo by means of green fluorescent protein (GFP) fusions. Several prnB–gfp gene fusions, driven by prnB native promoter sequences, were constructed and targeted to the genomic locus of a prnB null mutant. Chimeric proteins containing GFP fused to the C terminus of PrnB through a linker of two, four, or eight amino acids, with low potential to form secondary structure elements, were shown to be functional in vivo. A two-linker fusion results in partial complementation at both 25 and 37°C. A four-linker fusion affords almost full complementation at 25°C but partial complementation at 37°C, whereas the eight-linker fusion results in partial complementation at both temperatures but in no GFP fluorescence. These results show that the number of linker amino acids is critical for the correct expression and/or translocation of PrnB–GFP fused proteins to the plasma membrane and for the fluorescence of the GFP. The expression of the four-linker PrnB–GFP transporter was detected and analyzed in vivo by both conventional fluorescence and confocal laser microscopy. This chimeric protein is localized in the plasma membrane, secondarily in large vacuoles found in the swollen conidial end of the germlings, and in other small intracellular structures observed as fluorescent granules. A strong correlation between known patterns of PrnB expression and intensity of PrnB–GFP fluorescence was observed. This work also demonstrates that the GFP fusion technology is a unique tool with which to study the expression and cellular localization of low-abundance transmembrane transporters expressed from their native promoters.