Biology Reference
In-Depth Information
knockout mice which are deficient in DGAT2 die shortly after birth due to lipopenia,
a skin barrier defect that results in extreme dehydration. Unlike DGAT1 which is an
integral endoplasmic reticulum (ER) membrane protein, DGAT2 is located at both
the ER and LDs, where at the latter location it is hypothesized to play a role in LD
expansion (
Farese & Walther, 2009; Stone et al., 2004, 2009
). On oleate feeding,
DGAT2 can be seen on the LD surface. DGAT2 belongs to a separate family of en-
zymes that is less hydrophobic than DGAT1. Thus, when cells are fed oleate,
DGAT2 has been observed at the surface of the LD—a finding that is dependent
on its C-terminus—and may serve to catalyze TAG synthesis as an oligomeric com-
plex of 14-16 DGAT2 subunits (
McFie, Banman, Kary, & Stone, 2011
). These data
suggest that neutral lipid-synthesizing enzymes can be localized to both the ER and
LD surface.
While much information is known about the catabolism of LDs, little is under-
stood about the process of LD biogenesis. Numerous forward genetic screens have
demonstrated that up to 1.5% of the genomes of model organisms is important in the
regulation of fat storage. However, these studies have not found proteins involved
directly in the process of LD formation (
Ashrafi et al., 2003; Fei et al., 2008; Guo
et
al., 2008
). Thus far, screens have identified factors important for LD growth, main-
tenance, and trafficking.
FIT proteins were identified as targets of peroxisome proliferator-activated re-
ceptor
a
(PPAR-
a
) in the liver in an effort to identify proteins of unknown function
involved in intracellular fatty acid transport with respect to either TAG storage in
LDs or mitochondrial
b
-oxidation. PPAR-
a
is known to activate genes involved
in fatty acid oxidation, but counter-intuitively in mouse liver, pharmacological
activation of PPAR-
a
also induces genes involved in fatty acid biosynthesis, glycer-
olipid biosynthesis, and two perilipin (PLIN) family members Plin2 and Plin5.
Therefore, it was surmised that a microarray for PPAR-
a
targets might identify novel
proteins involved in LD biogenesis localized to the ER—the site of LD formation
(
Kadereit et al., 2008
). FIT1 and FIT2 were confirmed to be ER-localized, 292-
and 262-amino acid proteins, respectively, that are 50% similar and 35% identical.
However, these proteins do not share homology to any known proteins or protein
domains in higher eukaryotes, suggesting that they constituted a unique protein fam-
ily. Bioinformatics searches demonstrated that these proteins were phylogenetically
conserved as far back as
Saccharomyces cerevisiae
, which has two FIT2 orthologs,
SCS3 and yFIT2 (
Moir, Gross, Silver, & Willis, 2012
). FIT1 was conserved as far
back as the teleosts and through higher eukaryotes (
Hosaka, Nikawa, Kodaki, Ishizu,
&Yamashita, 1994; Young et al., 2010
). Mammalian FIT proteins have a differential
tissue expression pattern: FIT2 is ubiquitously expressed, with the highest levels
found in white and brown adipose tissue. High-level expression of FIT2 in adipose
tissue is regulated by PPAR-
g
(
Lefterova et al., 2008; Villanueva et al., 2011
). FIT1
is expressed primarily in oxidative tissues, with the highest levels in skeletal muscle
and lower levels in the heart (
Kadereit et al., 2008
). Both FIT1 and FIT2 were dem-
onstrated to have six transmembrane domains and localize to the ER, but not to LDs
(
Gross, Snapp, & Silver, 2010
).
