The major facilitator transporter Str3 is required for low-affinity heme acquisition in Schizosaccharomyces pombe

In the fission yeast Schizosaccharomyces pombe, acquisition of exogenous heme is largely mediated by the cell membrane-associated Shu1. Here, we report that Str3, a member of the major facilitator superfamily of transporters, promotes cellular heme import. Using a strain that cannot synthesize heme de novo (hem1 Delta) and lacks Shu1, we found that the heme-dependent growth deficit of this strain is rescued by hemin supplementation in the presence of Str3. Microscopic analyses of a hem1 Delta shu1 Delta str3 Delta mutant strain in the presence of the heme analog zinc mesoporphyrin IX (ZnMP) revealed thatZnMPfails to accumulate within the mutant cells. In contrast, Str3-expressing hem1 Delta shu1 Delta cells could take up ZnMP at a 10-mu M concentration. The yeast Saccharomyces cerevisiae cannot efficiently transport exogenously supplied hemin. However, heterologous expression of Str3 from S. pombe in S. cerevisiae resulted in ZnMP accumulation within S. cerevisiae cells. Moreover, heminagarose pulldown assays revealed that Str3 binds hemin. In contrast, an Str3 mutant in which Tyr and Ser residues of two putative heme-binding motifs ((YX3Y534)-Y-530 and (SX4Y557)-S-552) had been replaced with alanines exhibited a loss of affinity for hemin. Furthermore, this Str3 mutant failed to rescue the heme-dependent growth deficit of a hem1 Delta shu1 Delta str3 Delta strain. Further analysis by absorbance spectroscopy disclosed that a predicted extracellular loop region in Str3 containing the two putative hemebinding motifs interacts with hemin, with a K-D of 6.6 mu M. Taken together, these results indicate that Str3 is a second cell-surface membrane protein for acquisition of exogenous heme in S. pombe.