2.A.67 The Oligopeptide Transporter (OPT) Family

The OPT family consists of functionally well characterized oligopeptide (3-8 amino acid) transporters. One of the yeast homologues is the sexual differentiation process (ISP4) protein of Schizosaccharomyces pombe.  S. cerevisiae and S. pombe each possess three paralogues of the OPT family. Two transporters from S. cerevisiae, one from S. pombe, and one from Candida albicans have been functionally characterized, and all are peptide uptake systems. Homologues are also found in plants, bacteria and archaea (Gomolplitinant and Saier, 2011).

The prokaryotic homologues are very distant, being revealed only upon PSI-BLAST iterations, and they are also uncharacterized functionally. Energy coupling probable involves H⁺ symport. While the full-length yeast proteins are reported to be 700-900 residues long and exhibit 12 putative TMSs, bacterial homologue from H. influenzae is 633 amino acyl residues long and exhibits 15 putative TMSs (Gomolplitinant and Saier, 2011).

Eight opt genes encode putative oligopeptide transporters in Candida albicans (e.g. see OPT1, 2.A.67.1.1), almost all of which are represented by polymorphic alleles. opt 1,2,3Δ triple mutants have a severe growth defect, which was rescued by reintroduction of a single copy of OPT1, OPT2 or OPT3.The various oligopeptide transporters differ in their substrate preferences as shown by the ability of strains expressing specific opt genes to grow on peptides of defined length and sequence. These transporters can take up tri-octa peptides, each one with differing specificities (Reuss and Morschhauser, 2006).

An Fe³⁺-phytosiderophore uptake system of Zea mays, also known as 'yellow stripe1' (YS1) is encoded by the ys1 gene, the expression of which is increased in both roots and shoots under iron deficient conditions (Curie et al., 2001). When expressed in a mutant yeast lacking its native iron uptake system, it corrects the defect specifically in Fe³⁺-phytosiderophore media. YS1 has a glutamine-rich N-terminus that might function in Fe³⁺ binding. It has been shown to be a proton symporter that takes up phytosiderophore- and nicotianamine-chelated metals (Schaaf et al., 2004). Nicotianamine is a structural analogue of phytosiderophores and is also an intracellular metal chelator.

Nine genes (designated YS-like genes 1-9 or YS1-9) in Arabidopsis thaliana encode close homologues of YS1 (Curie et al., 2001). These proteins and nine other distant OPT family homologues are of about the same size (about 600 aas) and exhibit 12-13 putative TMSs. Some of these plant proteins may be more similar to the bacterial and archaeal homologues than to the yeast peptide porters. The plant homologues have been reviewed by Lubkowitz (2006).

The generalized transport reactions thought to be catalyzed by members of the OPT family are:

1) Oligopeptide (out) + nH⁺ (out) → oligopeptide (in) + nH⁺ (in).

2) Fe³⁺-phytosiderophore (out) + nH⁺ (out) → Fe³⁺-phytosiderophore (in) + nH⁺ (in)