Plastid fatty acid export (FAX) proteins inArabidopsis thaliana- the role of FAX1 and FAX3 in growth and development

Abstract

In plant cells, fatty acid (FA) synthesis occurs in the plastid stroma and thus requires subsequent FA export for lipid assembly in the endoplasmic reticulum. In this context, the membrane-intrinsic protein FAX1 has been described to mediate FA-export across the plastid inner envelope (IE). In Arabidopsis, FAX1 function is crucial for pollen cell wall formation, male fertility, cellular lipid homeostasis and plant biomass. Based on conserved structural features and sequence motifs, we here define the plant FAX-protein family localized in plastids. Besides their membrane-intrinsic domain, the plastid-targeted FAX1-FAX3 contain distinct N-terminal stretches. Among them, the apolipoprotein-like α-helical bundle of FAX2 is the most prominent. Further, we could unequivocally localize FAX2 and FAX3 proteins together with FAX1 to the IE membrane of chloroplasts and develop a topology model for FAX1, FAX2, and FAX3. In yeast, all plastid FAX proteins - i.e. FAX1, FAX2, FAX3, FAX4 - can complement for FA-transport function. For FAX1 we show a new function in pollen tube growth as well as together with FAX3 in seed/embryo development and in rosette leaf growth. Since in comparison to fax1 single knockout mutants, fax1/fax3 double knockouts are seedling lethal and not able to develop mature rosette leaves, we conclude that FAX1 and FAX3 function together in vegetative leaf growth.

Characterization of acyl-lipid thioesterase (ALT) enzymes from plants

“…2) (Dörmann et al, 1995;Jones et al, 1995). In Arabidopsis, the transmembrane proteins fatty acyl export 1-4 (FAX1-4) mediate fatty acid export from the plastid, but the precise mechanism by which this transport occurs is unknown (Li et al, 2015;Tian et al, 2019;Bugaeva et al, 2023). Traditional biological models hold that that free fatty acids are rapidly re-activated to acyl-CoA thioesters at the plastid outer envelope by an acyl-CoA synthetase, and are then mostly transported to the ER.…”