By binding to and inserting into the lipid bilayer, amphiphilic α-helices of proteins are involved in the curvature of biological membranes in all organisms. In particular, they are involved in establishing the complex membrane architecture of intracellular organelles like the endoplasmatic reticulum, Golgi apparatus, mitochondria, and chloroplasts. Thus, amphiphilic α-helices are essential for maintenance of cellular metabolism and fitness of organisms. Here we focus on the structure and function of membrane-intrinsic proteins, which are involved in membrane curvature by amphiphilic α-helices, in mitochondria and chloroplasts of the eukaryotic model organisms yeast and Arabidopsis thaliana. Further, we propose a model for transport of fatty acids and lipid compounds across the envelope of chloroplasts in which amphiphilic α-helices might play a role.

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.