The product of the SHR3 orthologue of Aspergillus nidulans has restricted range of amino acid transporter targets

The shrA gene of Aspergillus nidulans codes for a structural and functional homologue of Shr3p, a yeast ER membrane protein, which plays a crucial role in the secretory pathway of yeast amino acid permeases. shrA is a single-copy gene, whose expression is early activated during germination of A. nidulans conidiospores. ShrA is localized in the ER of the fungal cells and partially complements the shr3Δ phenotype. Differently from Saccharomyces cerevisiae, where SHr3p is necessary for membrane localization of the majority of amino acid permeases, deletion of the shrA locus in A. nidulans impairs a limited number of amino acid uptake activities, including those responsible for proline and aspartate transport. Strongly reduced membrane levels of a PrnB-sGFP fusion in a shrAΔ background clearly suggest a direct role of ShrA in the topogenesis of the proline specific transporter.