Biology Reference
In-Depth Information
INTRODUCTION
Lipid droplets (LDs) store and metabolize almost all neutral lipids in the cell, across
all species, and its formation and dynamics are directly linked to metabolic syn-
dromes in humans and to food production and biofuel development in other organ-
isms (
Farese &Walther, 2009; Martin & Parton, 2006; Murphy, 2011; Zehmer et al.,
2009
). Although it is an essential cellular organelle, compared to other similar cel-
lular organelles such as the mitochondria and endoplasmic reticulum (ER), our un-
derstanding of LDs still remains elusive, especially its biogenesis, dynamics, and
functions. Therefore, knowing its two only components, namely lipids and proteins,
is of critical importance. Studies of LD lipid and protein composition can be traced
back to 1970s. In the early stage, LDs were isolated and their lipids and proteins
analyzed from mammal tissues and cells, plants, and yeast (
Comai, Farber, &
Paulsrud, 1975; DiAugustine, Schaefer, & Fouts, 1973; Hood & Patton, 1973;
Jacks, Yatsu, & Altschul, 1967; Lang & Insull, 1970; Nissen & Bojesen, 1969;
Yatsu, Jacks, &Hensarling, 1971
). Information of both lipid and protein composition
provided by those studies is very limited, as lipidomics and proteomics were not yet
employed. In last decade the Human Genome Project and new proteomic technology
allowed proteomic study possible. Our group and other labs have isolated LDs and
performed proteomic analyses on them, and thereafter gained dramatic and useful
knowledge that proposes several putative roles for LDs according to their proteomes,
further evolving the classic definition of LDs, as a lipid inclusion to a functional
organelle (
Athenstaedt, Zweytick, Jandrositz, Kohlwein, & Daum, 1999; Bartz,
Zehmer, et al., 2007; Beller et al., 2006; Brasaemle, Dolios, Shapiro, & Wang,
2004; Cermelli, Guo, Gross, & Welte, 2006; Fujimoto et al., 2004; Jolivet et al.,
2004; Liu et al., 2004; Sato et al., 2006; Yang et al., 2012
).
Some of these proteomics findings have been supported by further functional stud-
ies. For example, LD-associated lipid synthetic enzymes identified from yeast indicate
that LD functions as one of the cellular lipid synthetic sites (
Athenstaedt et al., 1999
),
and further verified by
in vitro
lipid synthesis using isolated LDs. Moreover, the dis-
covery of Rab proteins and SNAREs in Chinese hamster ovary CHO K2 cells suggests
that LDs are involved in membrane trafficking (
Liu et al., 2004
), which has thereafter
been confirmed by Rab-mediated interactions between LDs and ER (
Martin, Driessen,
Nixon, Zerial, & Parton, 2005; Ozeki et al., 2005
), as well as LDs and endosome (
Liu
et al., 2007
). Furthermore, the discovery of proteins moving on and off LDs suggests
that LDs are very dynamic organelles (
Bartz, Zehmer, et al., 2007; Brasaemle et al.,
2004
). The identification of prostaglandin synthesis pathway shows that LDs produce
signal molecules (
Accioly et al., 2008
). The observation of ubiquitination-related pro-
teins suggests that LDs are involved in protein degradation (
Ploegh, 2007
), which has
been confirmed by the identification of LD-mediated HMG-CoA reductase recently
(
Hartman et al., 2010
). Lastly, the discovery of histone proteins in
Drosophila
LDs
suggests that LDs behave as a protein-storage depot (
Cermelli et al., 2006
).
The improvement of LD isolation methods (
Ding et al., 2013
) and the increas-
ing development of proteomic technologies play an essential role in the recent
