Tue, 19/11/2019 - 14:15
,
Campus SB, Building E2 6 Room E.04

Prof. Dr. Katrin Philippar
(
Host: Prof. Dr. Ludger Santen
)
ZHMB - Pflanzenbiologie, UdS (SB)

Structure and function of FAX/TMEM14 membrane proteins

Lipid metabolism in eukaryotic cells is compartmentalized into membrane-delimited organelles. These include chloroplasts for de novo fatty acid (FA) synthesis in plants, the endoplasmatic reticulum (ER) for assembly of membrane phospholipids and triacylglycerol (TAG) storage lipids as well as mitochondria (animals/yeast) and peroxisomes (plants) for FA breakdown by ß-oxidation. Further, TAG carbon energy lipids are packed in lipid droplet (LD) organelles, which originate from the ER and surround the hydrophobic TAG center by a lipid monolayer that contains several embedded proteins. For a proper function of cellular metabolism, therefore control of FA/lipid transport, biogenesis of and contacts between organelles and membranes as well as intracellular signaling and communication are essential. In plant cells, the protein FAX1 has been described to export FAs across the inner envelope membrane of chloroplasts and thereby FAX1 showed to be crucial for plant lipid homeostasis (Li et al., 2015). The function of orthologous proteins, which belong to the family of membrane-intrinsic TMEM14 proteins in vertebrates and yeast, however, is still enigmatic. TMEM14 proteins are predicted to insert into organellar membranes of chloroplasts, ER and mitochondria and contain four α-helical domains. The latter insert into the phospholipid bilayer either symmetrically or asymmetrically as hydrophobic or amphiphilic helices, respectively. Thereby, FAX/TMEM14 proteins appear to mediate bending and shaping of the membrane (Könnel et al., 2018). Further, under phosphate deficiency and/or cold stress, chloroplast FAX proteins might be involved in chloroplast-mitochondria contacts and lipid transfer. Interestingly, chloroplast FAX proteins contain an additional N-terminal region, which is absent in vertebrate and yeast TMEM14 relatives. The predicted secondary structure of this polypeptide stretch is very similar to human apolipoprotein E and insect apolipophorin. Thus, it is very likely that chloroplast FAX proteins contribute to membrane and organelle contacts as well as FA/lipid transport via their four α-helices and the N-terminal apolipoprotein-like domain.

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