Fat storage-inducing transmembrane (FIT or FITM) proteins are related to lipid phosphatase/phosphotransferase enzymes.

Matthew J Hayes,William A Prinz,Timothy P Levine,Namrata Ojha,Vineet Choudhary,John Jh Shin,George M Carman,Christopher Jr Loewen,Gil-Soo Han

doi:10.15698/mic2018.02.614

Abstract

Fat storage-inducing transmembrane (FIT or FITM) proteins have been implicated in the partitioning of triacylglycerol to lipid droplets and the budding of lipid droplets from the ER. At the molecular level, the sole relevant interaction is that FITMs directly bind to triacyglycerol and diacylglycerol, but how they function at the molecular level is not known. Saccharomyces cerevisiae has two FITM homologues: Scs3p and Yft2p. Scs3p was initially identified because deletion leads to inositol auxotrophy, with an unusual sensitivity to addition of choline. This strongly suggests a role for Scs3p in phospholipid biosynthesis. Looking at the FITM family as widely as possible, we found that FITMs are widespread throughout eukaryotes, indicating presence in the last eukaryotic common ancestor. Protein alignments also showed that FITM sequences contain the active site of lipid phosphatase/phosphotransferase (LPT) enzymes. This large family transfers phosphate-containing headgroups either between lipids or in exchange for water. We confirmed the prediction that FITMs are related to LPTs by showing that single amino-acid substitutions in the presumptive catalytic site prevented their ability to rescue growth of the mutants on low inositol/high choline media when over-expressed. The substitutions also prevented rescue of other phenotypes associated with loss of FITM in yeast, including mistargeting of Opi1p, defective ER morphology, and aberrant lipid droplet budding. These results suggest that Scs3p, Yft2p and FITMs in general are LPT enzymes involved in an as yet unknown critical step in phospholipid metabolism.

Highlights

All eukaryotic cells store the neutral lipids triacylglycerol and sterol ester in lipid droplets [1]
By light microscopy these puncta co-localised with bodies that were highly refractile by differential interference contrast (DIC), a characteristic of lipid droplets
This quantity is not affected by fat storage-induced transmembrane (FITM) deletion [21] instead, a more subtle effect is that FITM proteins are required for correct budding of nascent lipid droplets, a phenotype that is conserved in human cells [26]

Summary

Introduction

All eukaryotic cells store the neutral lipids triacylglycerol and sterol ester in lipid droplets [1] Understanding this is societally important both for feeding the growing human population [2] and to address problems of the consumption of excess calories with associated metabolic syndrome, type 2 diabetes and atherosclerosis [3]. FITM1 is mainly restricted to skeletal muscle, whereas FITM2 is expressed in most tissues, especially in adipose tissue They are required for triacylglycerol storage in lipid droplets [5]. Homozygous FITM2 mutations introducing a nonsense codon very near to the first residue cause deafness and dystonia [9] This clinical picture is at odds with the effect of homozygous deletion of FITM2 in mice, which causes problems mainly through lack of lipid droplet formation [10, 11]

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Conclusion