Higher Plant Plastids and Cyanobacteria Have Folate Carriers Related to Those of Trypanosomatids

Abstract

Cyanobacterial and plant genomes encode proteins with some similarity to the folate and biopterin transporters of the trypanosomatid parasite Leishmania. The Synechocystis slr0642 gene product and its closest Arabidopsis homolog, the At2g32040 gene product, are representative examples. Both have 12 probable transmembrane domains, and the At2g32040 protein has a predicted chloroplast transit peptide. When expressed in Escherichia coli pabA pabB or folE, mutants, which are unable to produce or take up folates, the slr0642 protein and a modified At2g32040 protein (truncated and fused to the N terminus of slr0642) enabled growth on 5-formyltetrahydrofolate or folic acid but not on 5-formyltetrahydrofolate triglutamate, demonstrating that both proteins mediate folate monoglutamate transport. Both proteins also mediate transport of the antifolate analogs methotrexate and aminopterin, as evidenced by their ability to greatly increase the sensitivity of E. coli to these inhibitors. The full-length At2g32040 polypeptide was translocated into isolated pea chloroplasts and, when fused to green fluorescent protein, directed the passenger protein to the envelope of Arabidopsis chloroplasts in transient expression experiments. At2g32040 transcripts were present at similar levels in roots and aerial organs, indicating that the protein occurs in non-green plastids as well as chloroplasts. Insertional inactivation of At2g32040 significantly raised the total folate content of chloroplasts and lowered the proportion of 5-methyltetrahydrofolate but did not discernibly affect growth. These findings establish conservation of function among folate and biopterin transporter family proteins from three kingdoms of life.

The Folylpolyglutamate Synthetase Plastidial Isoform Is Required for Postembryonic Root Development in Arabidopsis

“…9, B and D). In this regard, it is worth noting that the main form of folate found in chloroplasts is polyglutamylated 5,10-CH=THF, which comprises both 5,10-CH=THF and 10-CHO-THF (Klaus et al, 2005;Orsomando et al, 2005, Mehrshahi et al, 2010. In Arabidopsis, de novo purine biosynthesis occurs in plastids and requires 10-CHO-THF in two transformylation steps (Zrenner et al, 2006).…”

“…1). Polyglutamylation is also essential for the retention of folates within a given cellular compartment, since organellar folate transporters are more specific to monoglutamylated folates (Bedhomme et al, 2005;Klaus et al, 2005;Raichaudhuri et al, 2009). In Arabidopsis, three genes, AtDFB, AtDFC, and AtDFD, have been reported to encode the plastidial, mitochondrial, and cytoplasmic FPGS isoforms, respectively (Ravanel et al, 2001; Fig.…”

ArabidopsisSAMT1 Defines a Plastid Transporter Regulating Plastid Biogenesis and Plant Development

“…For a given substrate, one or multiple transport systems might be involved. For example, plastids seemingly possess two independent and structurally unrelated transport systems for folates, one belonging to the MCF (Bedhomme et al, 2005) and the other being related to that of trypanosomes and cyanobacteria (Klaus et al, 2005). In addition, compensatory mechanisms have been observed for plastid transporters (reviewed in Weber et al, 2004Weber et al, , 2005.…”

The Import and Export Business in Plastids: Transport Processes across the Inner Envelope Membrane

“…1; Table I; Tyra et al, 2007). In contrast, only 12% of the membrane proteins were contributed by the cyanobacterial endosymbiont, for example a second folate transporter (Klaus et al, 2005). For about 20% of the proteins their origin could not be conclusively determined.…”