More Transporters, More Substrates: The Arabidopsis Major Facilitator Superfamily Revisited

Abstract

The Major Facilitator Superfamily (MFS) is ubiquitous in living organisms and represents the largest group of secondary active membrane transporters. In plants, significant research efforts have focused on the role of specific families within the MFS, particularly those transporting macronutrients (C, N, and P) that constitute the vast majority of the members of this superfamily. Other MFS families remain less explored, although a plethora of additional substrates and physiological functions have been uncovered. Nevertheless, the lack of a systematic approach to analyzing the MFS as a whole has obscured the high diversity and versatility of these transporters. Here, we present a phylogenetic analysis of all annotated MFS domaincontaining proteins encoded in the Arabidopsis thaliana genome and propose that this superfamily of transporters consists of 218 members, clustered in 22 families. In reviewing the available information regarding the diversity in biological functions and substrates of Arabidopsis MFS members, we provide arguments for intensified research on these membrane transporters to unveil the breadth of their physiological relevance, disclose the molecular mechanisms underlying their mode of action, and explore their biotechnological potential.

The Tapetal Major Facilitator NPF2.8 Is Required for Accumulation of Flavonol Glycosides on the Pollen Surface in Arabidopsis thaliana

“…The characteristic formation of Hechtian strands during plasmolysis was observed: under conditions of concave plasmolysis, the protoplast is connected to neighboring cells by narrow cytoplasmatic strings only ( Figure 7E). These data confirm that FST1 indeed is a plasma membrane-localized putative transporter, consistent with reports for most members of the NPF family with the exception of NPF2.9 from periwinkle (Nour-Eldin et al, 2012;Niño-Gonzáles et al, 2019).…”

“…We termed this gene FST1 (for FLAVONOL SOPHOROSIDE TRANSPORTER 1). According to the recent nomenclature of the NPF family (Léran et al, 2014), it is identical to NPF2.8 referred to by Jørgensen et al (2017) and Niño-Gonzáles et al (2019).…”

“…In Arabidopsis, this family comprises 218 transmembrane proteins, classified into 22 subfamilies. FST1, termed NPF2.8 was placed into the heterogeneous NPF2 subfamily and clusters with the glucosinolate transporters NPF2.9, NPF2.10 and NPF2.11 (Jørgensen et al 2017;Niño-Gonzáles et al, 2019) but showed only weak glucosinolate transport activity. Using the amino acid sequence of FST1 in a BLAST (Basic Local Alignment Search Tool) against the protein database (https://blast.ncbi.nlm.nih.gov/Blast.cgi), the sequence identity of NPF2.8 to the characterized glucosinolate transporters was only 38% and thus less significant than anticipated.…”

“…Two types of nitrate (NTR) and peptide (PTR) transporters have been described in plants and show partially overlapping specificities (Tsay et al, 2007). They are members of the universal major facilitator superfamily (MFS) of transporters (http://pfam.xfam.org/clan/MFS; Marger and Saier Jr., 1993;Yan, 2015, Niño-Gonzáles et al, 2019. A more universal nomenclature and rigid classification of the substrate-based nomenclature of this nitrate and peptide family (now termed NPF) was proposed by Léran et al (2014), identifying eight subfamilies in 31 genomes at the time.…”

“…A more universal nomenclature and rigid classification of the substrate-based nomenclature of this nitrate and peptide family (now termed NPF) was proposed by Léran et al (2014), identifying eight subfamilies in 31 genomes at the time. In Arabidopsis, an astonishing number of 60 NPF-type transporters were identified in three families within the MFS superfamily (Corratgé-Faillie and Lacombe, 2017;Niño-Gonzáles et al, 2019;Patil, 2019). Using a functional genomics approach, Nour-Eldin et al (2012) demonstrated that the two plasma membrane-localized and proton gradient-driven NTR-type GLUCOSINOLATE TRANSPORTER GTR1 and GTR2, also termed NPF2.10 and NPF2.11, respectively, were required for long-distance translocation of aliphatic and indole glucosinolates, while GTR3 (NPF2.9) was characterized as an indole glucosinolate transporter (Jørgensen et al, 2017).…”