Biology Reference
In-Depth Information
A recent review compares the state of the art of LD research ranging from archaea
to mammals with emphasis on the yeast
S. cerevisiae
as an appropriate model system
(
Murphy, 2012
). Importantly, a number of parallels between LD from yeast and
mammalian cells have been discovered supporting this view. Examples for such par-
allels are the occurrence of lipid-metabolizing enzymes in both types of LD, such as
the lipases ATGL and Tgl3p, Tgl4p, and Tgl5p, respectively (
Athenstaedt & Daum,
2003, 2005; Zimmermann et al., 2004
), enzymes of sterol biosynthesis (
Caldas &
Herman, 2003; Leber et al., 1998; Milla et al., 2002; Ohashi, Mizushima, Kabeya,
& Yoshimori, 2003; Van Meer, 2001; Zinser, Paltauf, & Daum, 1993
), or more spe-
cifically seipin in mammalian cells and Fld1p in yeast (
Fei, Du, & Yang, 2011; Fei
et al., 2008; Wolinski, Kolb, Hermann, Koning, & Kohlwein, 2011
). Also in plant
LD certain enzymes of lipid metabolism were detected (for review see
Baud &
Lepiniec, 2010; Murphy, 2001
). The major advantage of the yeast, however, to per-
form studies with LD or other organelles, is the ease of manipulation either by culture
conditions or by genetic means.
The main storage lipids of the yeast are triacylglycerols (TG) and steryl esters
(SE). These biologically inert forms of free fatty acids (FA) and sterols are often
referred to as nonpolar or neutral lipids as they lack charged groups. They mainly
function as a reservoir of energy and building blocks for membrane components,
but at the same time they provide an internal cell protective mechanism against pos-
sible toxic effects caused by an excess of free FA and sterols. LD consist of a highly
hydrophobic core of mainly TG, which is surrounded by shells of SE and covered by
a phospholipid monolayer (
Czabany et al., 2008
) with specific proteins embedded in
the surface membrane of LD (
Athenstaedt, Zweytick, Jandrositz, Kohlwein, &
Daum, 1999; Czabany, Athenstaedt, & Daum, 2007; Leber, Zinser, Zellnig,
Paltauf, & Daum, 1994
). Although LD appear to be important for yeast cells under
normal growth conditions their existence is not essential (
Sandager et al., 2002
).
The biogenesis of LD is still a matter of discussion (
Kohlwein, Veenhuis, & Van
der Klei, 2013
). However, all biogenesis models have in common that LD are most
likely formed
de novo
from the endoplasmic reticulum (ER) (
Walther & Farese,
2012
). The currently most accepted model of LD biogenesis proposes formation
at specific membrane microdomains in the ER where nonpolar lipids accumulate un-
til the size of the LD reaches a critical dimension to bud off forming an independent
organelle-like structure (
Murphy & Vance, 1999; Ploegh, 2007; W¨ltermann et al.,
2005; Zweytick et al., 2000
). It has to be noted that LD do not only serve as lipid
storage pool but also fulfill many other functions in lipid metabolism (
Zinser
et al., 1993
). As an example,
Connerth et al. (2010)
discussed an indirect role of
LD in maintaining ideal membrane fluidity under environmental stress caused by
exogenous FA. Besides the undisputed influence of LD on lipid homeostasis, func-
tions which are unrelated to lipid turnover have emerged such as storage and degra-
dation of protein aggregates and incorrectly folded proteins (
Fei, Wang, Fu, Bielby,
& Yang, 2009; Fujimoto, Ohsaki, Cheng, Suzuki, & Shinohara, 2008
). Recent
research on the interaction of LD with other organelles, that is, the ER (
Fei et al.,
2009; Jacquier et al., 2011; Wolinski et al., 2011
), peroxisomes (
Binns et al.,
