The Organizing Potential of Sphingolipids in Intracellular Membrane ...
The Organizing Potential of Sphingolipids in Intracellular Membrane ...
The Organizing Potential of Sphingolipids in Intracellular Membrane ...
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
1702 HOLTHUIS, POMORSKI, RAGGERS, SPRONG, AND VAN MEER<br />
medial Golgi and <strong>in</strong>volves the transfer <strong>of</strong> phosphochol<strong>in</strong>e<br />
from PC to ceramide, yield<strong>in</strong>g diacylglycerol as a (potentially<br />
important) side product (101, 150). A second SM<br />
synthase activity has been located on the cell surface<br />
(392). Two SM synthases with different properties have<br />
also been found <strong>in</strong> the <strong>in</strong>tra-erythrocyte stage <strong>of</strong> Plasmodium<br />
(120). <strong>The</strong> SM synthases rema<strong>in</strong> to be identified.<br />
<strong>The</strong> ceramide:glucosyltransferase <strong>in</strong>volved <strong>in</strong> GlcCer production<br />
is located on the cytosolic surface <strong>of</strong> the early<br />
Golgi (45, 65, 100, 151, 224). Whereas the GlcCer synthase<br />
<strong>in</strong> mammals is encoded by a s<strong>in</strong>gle gene, three GlcCer<br />
synthase analogs have been identified <strong>in</strong> the C. elegans<br />
genome (144). <strong>The</strong> physiological significance <strong>of</strong> this multiplicity<br />
rema<strong>in</strong>s to be solved. By an unknown mechanism,<br />
GlcCer is translocated to the Golgi lumen where it can be<br />
trapped by galactosylation to Galß1–4GlcCer (LacCer;<br />
Refs. 45, 183, 267). Various series <strong>of</strong> complex glycosph<strong>in</strong>golipids<br />
can then be generated by stepwise addition <strong>of</strong><br />
sugars to LacCer, whereby the first sugar and its glycosidic<br />
l<strong>in</strong>kage determ<strong>in</strong>e the name <strong>of</strong> the series (53), e.g.,<br />
Gal1–4: globo- or Gb, Gal1–3: isoglobo or iGb, Glc-<br />
NAcß1–3: lacto or Lc (Fig. 3). <strong>The</strong> ganglio series (or Gg)<br />
is based on GalNAcß1–4LacCer. <strong>The</strong> ganglio series also<br />
comprises the simple gangliosides NeuAc 1–3-LacCer, wellknown<br />
under the Svennerholm nomenclature as GM3,<br />
GD3, and GT3, respectively (53). Glycosph<strong>in</strong>golipids <strong>in</strong><br />
the various series may be fucosylated or sulfated. LacCer<br />
synthesis and all further conversions occur <strong>in</strong> the lumen<br />
<strong>of</strong> the Golgi and require import <strong>of</strong> the necessary sugar<br />
nucleotides (45, 183; reviewed <strong>in</strong> Ref. 209). With the use <strong>of</strong><br />
subcellular fractionation, a sequential distribution over<br />
the Golgi was observed for glycosph<strong>in</strong>golipid glycosyltransferases<br />
(143, 380, 381), with a significant overlap <strong>in</strong><br />
distributions. A later paper (184) assigned the synthesis <strong>of</strong><br />
LacCer, GM3, and GM2 to the trans-Golgi and the TGN,<br />
whereby a considerable fraction <strong>of</strong> the LacCer and GM3<br />
synthases localized to the cis-Golgi. Pharmacological<br />
studies, us<strong>in</strong>g the drugs brefeld<strong>in</strong> A (308, 333, 384, 427)<br />
and monens<strong>in</strong> (244, 316, 317, 385) to discrim<strong>in</strong>ate enzymes<br />
<strong>of</strong> the Golgi stack from those <strong>in</strong> the TGN, localized<br />
GM2 synthase (GalNAc-transferase) and two galactosyltransferases<br />
<strong>of</strong> complex glycosph<strong>in</strong>golipid synthesis to<br />
the TGN (see, however, Ref. 426). <strong>The</strong> same conclusion<br />
was reached <strong>in</strong> a study on mitotic cells (62).<br />
Arthropods and mollusks transfer a mannose onto<br />
GlcCer and further extend this cha<strong>in</strong> <strong>in</strong> the arthro (At) or<br />
the mollu (Mu) series. In some mammalian tissues, notably<br />
myel<strong>in</strong>, but <strong>in</strong> humans also the epithelia <strong>of</strong> the gastro<strong>in</strong>test<strong>in</strong>al<br />
and urogenital tracts, ceramide is ma<strong>in</strong>ly glycosylated<br />
to GalCer by the ceramide:galactosyltransferase,<br />
an enzyme situated on the lum<strong>in</strong>al face <strong>of</strong> the ER (328,<br />
359, 360). <strong>The</strong> enzyme displays a preference for ceramides<br />
conta<strong>in</strong><strong>in</strong>g a 2-hydroxylated fatty acid which are abundant<br />
<strong>in</strong> these tissues. GalCer can be further galactosylated and/or<br />
sulfated (or sialylated) <strong>in</strong> the Golgi lumen (45, 183, 373).<br />
Physiol Rev • VOL 81 • OCTOBER 2001 • www.prv.org<br />
3. Synthesis <strong>of</strong> <strong>in</strong>ositol sph<strong>in</strong>golipids<br />
In contrast to animals, all fungi and plants studied<br />
s<strong>of</strong>ar, as well as several protozoa, add <strong>in</strong>ositol phosphate<br />
to phytoceramide to form IPC (192). <strong>The</strong> biosynthetic<br />
route <strong>of</strong> <strong>in</strong>ositol sph<strong>in</strong>golipids <strong>in</strong> yeast has been studied <strong>in</strong><br />
great detail. Yeast produces only three types <strong>of</strong> <strong>in</strong>ositol<br />
sph<strong>in</strong>golipids: IPC, mannose 1–2IPC: MIPC, and <strong>in</strong>ositol-<br />
1-P-6 mannose 1–2IPC or mannose-(<strong>in</strong>ositol-P) 2-ceramide:<br />
M(IP) 2C. <strong>The</strong> IPC synthase, or an essential subunit<br />
there<strong>of</strong>, is encoded by the AUR1 gene (259). AUR1 homologs<br />
have been identified <strong>in</strong> a wide variety <strong>of</strong> fungi<br />
(132, 176). IPC activity is effectively blocked by the antifungal<br />
agents aureobasid<strong>in</strong> A (259), khafrefung<strong>in</strong> (219),<br />
and rustmyc<strong>in</strong> (218). Because IPC synthase activity is<br />
essential and not found <strong>in</strong> mammals, it provides an ideal<br />
target for therapeutic drugs to fight pathogenic fungi <strong>in</strong><br />
immunocompromised <strong>in</strong>dividuals. IPC synthesis <strong>in</strong> yeast<br />
was previously thought to occur <strong>in</strong> the ER. This idea was<br />
based on the fact that the formation <strong>of</strong> IPC from ERderived<br />
ceramide and PI cont<strong>in</strong>ues under conditions<br />
when ER-to-Golgi vesicular transport is blocked <strong>in</strong> temperature-sensitive<br />
secretion (sec) mutants (292). However,<br />
fluorescence microscopy and membrane fractionation<br />
experiments have recently shown that both the<br />
AUR1 gene product and IPC synthase activity are located<br />
<strong>in</strong> the Golgi (195). This discrepancy <strong>in</strong> results rema<strong>in</strong>s to<br />
be clarified but could be expla<strong>in</strong>ed if the IPC synthase<br />
would constitutively cycle between the ER and the Golgi,<br />
a feature <strong>in</strong>herent <strong>of</strong> several Golgi-based prote<strong>in</strong>s (60, 146,<br />
210). An alternative explanation could be that ceramide<br />
made <strong>in</strong> the ER can reach the Golgi by a vesicle-<strong>in</strong>dependent<br />
transport mechanism (see sect. VC).<br />
Whereas the sidedness <strong>of</strong> IPC production is unknown,<br />
mannosylation to form MIPC most likely occurs<br />
<strong>in</strong> the lumen <strong>of</strong> the Golgi. <strong>The</strong> latter reaction requires at<br />
least three genes: SUR1, VRG4, and CSG2. SUR1 most<br />
likely encodes a mannosyltransferase (16), whereas VRG4<br />
is required for GDP-mannose transport <strong>in</strong>to the Golgi<br />
lumen (68). CSG2 encodes a member <strong>of</strong> the major facilitator<br />
superfamily, and its role <strong>in</strong> MIPC production is<br />
unclear (432). <strong>The</strong> f<strong>in</strong>al and most abundant sph<strong>in</strong>golipid<br />
<strong>in</strong> yeast, M(IP) 2C, is formed by transfer <strong>of</strong> <strong>in</strong>ositol phosphate<br />
from PI onto MIPC and requires a prote<strong>in</strong> encoded<br />
by the IPT1 gene (77). This reaction resembles the one<br />
that yields IPC. Accord<strong>in</strong>gly, the IPT1 and AUR1 gene<br />
products exhibit a strik<strong>in</strong>g structural similarity.<br />
4. Sph<strong>in</strong>golipid hydrolysis and signal<strong>in</strong>g<br />
sph<strong>in</strong>golipids<br />
<strong>The</strong> major pathway along which sph<strong>in</strong>golipids are<br />
degraded is removal <strong>of</strong> the head group and subsequent<br />
hydrolysis <strong>of</strong> ceramide to sph<strong>in</strong>goid base and free fatty<br />
acid. <strong>The</strong> breakdown products are then further metabolized<br />
or reutilized. Glycosph<strong>in</strong>golipids are degraded via a