The Organizing Potential of Sphingolipids in Intracellular Membrane ...
The Organizing Potential of Sphingolipids in Intracellular Membrane ...
The Organizing Potential of Sphingolipids in Intracellular Membrane ...
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1710 HOLTHUIS, POMORSKI, RAGGERS, SPRONG, AND VAN MEER<br />
TGN (114; Fig. 4). 28 Preferential <strong>in</strong>teractions between lipids<br />
and transmembrane prote<strong>in</strong>s with the best match<strong>in</strong>g hydrophobic<br />
length has been recognized as a potential mechanism<br />
for prote<strong>in</strong> sort<strong>in</strong>g <strong>in</strong> the Golgi (32, 255). This idea emerged<br />
with the observation that the transmembrane doma<strong>in</strong>s<br />
(TMDs) <strong>of</strong> Golgi-resident prote<strong>in</strong>s are on average five residues<br />
shorter than those <strong>of</strong> plasma membrane prote<strong>in</strong>s<br />
(32). 29 This phenomenon is well conserved among eukaryotic<br />
cells, from mammals (32) to yeast (195). It has been<br />
postulated that due to their shorter TMDs, Golgi-resident<br />
prote<strong>in</strong>s would be excluded from the thicker sph<strong>in</strong>golipid/<br />
sterol-enriched membrane regions dest<strong>in</strong>ed for the cell surface<br />
and hence be reta<strong>in</strong>ed <strong>in</strong> the Golgi (32, 255). 30 In support<br />
<strong>of</strong> this idea, lengthen<strong>in</strong>g the TMDs <strong>of</strong> several Golgi<br />
enzymes results <strong>in</strong> their movement to the plasma membrane<br />
(229, 253, 254), while shorten<strong>in</strong>g the TMD <strong>of</strong> a plasma membrane<br />
prote<strong>in</strong> causes its accumulation <strong>in</strong> the Golgi (60). In<br />
addition, the ER residency <strong>of</strong> several membrane prote<strong>in</strong>s<br />
can be disrupted by elongat<strong>in</strong>g their TMDs (282, 422). TMD<br />
length has also been recognized as a critical parameter <strong>in</strong> the<br />
sort<strong>in</strong>g <strong>of</strong> SNAREs (298), membrane prote<strong>in</strong>s whose specific<br />
<strong>in</strong>teractions contribute significantly to the specificity <strong>of</strong> vesicle<br />
dock<strong>in</strong>g and fusion (236, 311). Particularly <strong>in</strong>truig<strong>in</strong>g <strong>in</strong><br />
this respect is the f<strong>in</strong>d<strong>in</strong>g that the requirement <strong>of</strong> exocytic<br />
v-SNAREs (vesicular SNAREs) for secretion <strong>in</strong> yeast can be<br />
28 <strong>The</strong> shape <strong>of</strong> the thickness gradient is not known. In one extreme<br />
model, the phase separation between sph<strong>in</strong>golipids/cholesterol<br />
and unsaturated glycerophospholipids gives rise to a thick doma<strong>in</strong> that<br />
is transported to the plasma membrane and a th<strong>in</strong> doma<strong>in</strong> that is<br />
transported back to the ER. However, it is clear from the literature that<br />
glycerolipid membranes become thicker upon addition <strong>of</strong> cholesterol<br />
(see footnote 4). Also, it is clear that sph<strong>in</strong>golipid-rich membranes <strong>of</strong><br />
different thickness can leave the TGN (like the apical and basolateral<br />
transport vesicles <strong>in</strong> epithelial cells). We therefore hypothesize that the<br />
membrane thickness <strong>in</strong>creases gradually across the Golgi stack. Reasons<br />
for this may be the presence <strong>in</strong> cis-Golgi cisternae <strong>of</strong> sph<strong>in</strong>golipids<br />
at a concentration that is <strong>in</strong>sufficient to cause phase separation, a<br />
gradient <strong>in</strong> sph<strong>in</strong>golipid synthesis, and a cholesterol gradient. However,<br />
a proper understand<strong>in</strong>g <strong>of</strong> this phenomenon will require a quantitative<br />
analysis <strong>of</strong> these parameters.<br />
29 Previous studies had revealed that the steady-state localization<br />
<strong>of</strong> <strong>in</strong>tra-Golgi process<strong>in</strong>g enzymes is largely determ<strong>in</strong>ed not by coatb<strong>in</strong>d<strong>in</strong>g<br />
motifs but by their TMDs. In all cases exam<strong>in</strong>ed s<strong>of</strong>ar, the TMDs<br />
<strong>of</strong> glycosyltransferases are sufficient to direct localization <strong>of</strong> hybrid<br />
prote<strong>in</strong>s to the Golgi (reviewed <strong>in</strong> Ref. 109). <strong>The</strong>se TMDs lack obvious<br />
sequence motifs that could serve as sort<strong>in</strong>g determ<strong>in</strong>ants, and attempts<br />
to identify such motifs by mutagenesis have failed.<br />
30 It is important to note that the extent to which lipids <strong>in</strong> a bilayer<br />
can distort to match the length <strong>of</strong> a transmembrane -helix <strong>in</strong> a prote<strong>in</strong><br />
is limited. Although a short transmembrane -helix can <strong>in</strong>corporate <strong>in</strong>to<br />
a th<strong>in</strong> bilayer, it cannot <strong>in</strong>corporate <strong>in</strong>to a bilayer that is too thick, and<br />
<strong>in</strong>stead will form aggregates (164, 190). A long transmembrane -helix,<br />
on the other hand, can <strong>in</strong>corporate <strong>in</strong>to either a thick or a th<strong>in</strong> bilayer,<br />
<strong>in</strong> the latter case by tilt<strong>in</strong>g. Tilt<strong>in</strong>g <strong>of</strong> a transmembrane prote<strong>in</strong> may alter<br />
its conformation and function. <strong>The</strong> effect <strong>of</strong> bilayer thickness on prote<strong>in</strong><br />
function will probably be stronger <strong>in</strong> the case <strong>of</strong> prote<strong>in</strong>s conta<strong>in</strong><strong>in</strong>g a<br />
bundle <strong>of</strong> multiple transmembrane helices (e.g., an ion channel) as a<br />
slid<strong>in</strong>g <strong>of</strong> helices with<strong>in</strong> a bundle may be neccessary to achieve the most<br />
optimal hydrophobic match. Interest<strong>in</strong>gly, a coupl<strong>in</strong>g between hydrophobic<br />
membrane thickness and prote<strong>in</strong> activity has been observed for<br />
several transporter prote<strong>in</strong>s, <strong>in</strong>clud<strong>in</strong>g ion pumps (63, 362).<br />
Physiol Rev • VOL 81 • OCTOBER 2001 • www.prv.org<br />
bypassed by genetic mutations <strong>in</strong> enzymes responsible for<br />
the elongation <strong>of</strong> the long-cha<strong>in</strong> fatty acids found <strong>in</strong> ceramide<br />
and sph<strong>in</strong>golipids (67). 31<br />
Apart from be<strong>in</strong>g short, some Golgi TMDs display an<br />
unusual high content <strong>of</strong> the bulky residue phenylalan<strong>in</strong>e, a<br />
feature that may well promote a segregation from the ordered<br />
sph<strong>in</strong>golipid/sterol-rich membrane regions (32). Oligomerization<br />
<strong>in</strong>to multi-enzyme complexes (265) provides<br />
another factor that could <strong>in</strong>fluence phase behavior <strong>of</strong> Golgi<br />
enzymes. Collectively, the variation <strong>in</strong> such properties would<br />
<strong>of</strong>fer considerable scope for establish<strong>in</strong>g different steadystate<br />
distributions <strong>of</strong> enzymes with<strong>in</strong> the Golgi stack. Bidirectional<br />
transport mediated by percolat<strong>in</strong>g COPI vesicles or<br />
transient tubular connections would allow an enzyme to<br />
transiently explore the entire stack until it f<strong>in</strong>ds a cisterna<br />
whose lipid composition suits its TMD (283). Ongo<strong>in</strong>g sph<strong>in</strong>golipid<br />
synthesis and depletion <strong>of</strong> unsaturated glycerolipids<br />
will gradually remodel the lipid composition <strong>of</strong> the cisternal<br />
bilayer to that <strong>of</strong> the plasma membrane, and eventually drive<br />
all Golgi-resident prote<strong>in</strong>s, <strong>in</strong>clud<strong>in</strong>g the sph<strong>in</strong>golipid-synthesiz<strong>in</strong>g<br />
enzymes themselves, <strong>in</strong>to recycl<strong>in</strong>g COPI vesicles.<br />
Likewise, plasma membrane prote<strong>in</strong>s whose rapid<br />
diffusion through the stack is mediated by percolat<strong>in</strong>g<br />
vesicles or tubules could achieve the desired directionality<br />
<strong>in</strong> transport if their membrane anchors would favor<br />
cisternal bilayers that have become thicker and more<br />
organized due to a progressive accumulation <strong>of</strong> sph<strong>in</strong>golipids<br />
and sterols. Indeed, GPI-anchored prote<strong>in</strong>s have a<br />
high aff<strong>in</strong>ity for glycosph<strong>in</strong>golipid/cholesterol-rich doma<strong>in</strong>s<br />
(see sect. IVB), a feature thought to be required for<br />
their efficient transport from the Golgi to the cell surface<br />
(233, 345). <strong>The</strong> same has been reported for <strong>in</strong>fluenza virus<br />
hemagglut<strong>in</strong><strong>in</strong> (see sect. IVB; Ref. 161).<br />
<strong>The</strong> notions that sort<strong>in</strong>g <strong>in</strong> the Golgi depends on transitions<br />
<strong>in</strong> bilayer thickness and that these transitions are the<br />
result <strong>of</strong> sph<strong>in</strong>golipid-<strong>in</strong>duced phase separations provide a<br />
compell<strong>in</strong>g explanation for why structural features render<br />
sph<strong>in</strong>golipids essential for the viabilty <strong>of</strong> cells; they form an<br />
<strong>in</strong>tegral part <strong>of</strong> the mechanisms by which cells generate the<br />
compositional and functional differences between their<br />
plasma membrane and <strong>in</strong>ternal organelles.<br />
2. <strong>Organiz<strong>in</strong>g</strong> specialized export sites<br />
for polarized secretion<br />
<strong>The</strong> TGN is a major branch<strong>in</strong>g po<strong>in</strong>t <strong>in</strong> secretory<br />
traffic where apical and basolateral cell surface compo-<br />
31 <strong>Membrane</strong> fusion reconstitution experiments have revealed that<br />
v-SNAREs active <strong>in</strong> the lysosomal system <strong>of</strong> yeast can replace the<br />
exocytic v-SNAREs <strong>in</strong> a reaction that mimics fusion <strong>of</strong> secretory vesicles<br />
with the plasma membrane (236). With respect to the observed redundancy<br />
<strong>of</strong> exocytic v-SNAREs <strong>in</strong> a fatty acid elongation mutant (67), it is<br />
tempt<strong>in</strong>g to speculate that a shorten<strong>in</strong>g <strong>of</strong> sph<strong>in</strong>golipid-associated fatty<br />
acids would cause lysosomal v-SNAREs to leak <strong>in</strong>to secretory vesicles<br />
and take over the function <strong>of</strong> the exocytic v-SNAREs.