Lipid Rafts and the Cytoskeleton

Lipid Rafts, Signalling and the Cytoskeleton

Lipid rafts are specialised membrane domains enriched in certain lipids cholesterol and proteins. The existence of lipid rafts was first hypothesised in 1988 (Simons & van Meer, 1988; Simon & Ikonen, 1997), but what we know as "caveolae" were first observed much earlier (Palade, 1953; Yamada, 1955). Caveolae are flask shaped invaginations on the cell surface that are a type of membrane raft, these were named "caveolae intracellulare" (Yamada, 1955). After a long argument (Jacobson & Dietrich, 1999), most now consider that these rafts actually exist, however, there is some confusion surrounding the classification of these rafts. It presently seems that there could be three types; caveolae, glycosphingolipid enriched membranes (GEM), and polyphospho inositol rich rafts. It may also be that there are inside rafts (PIP₂ rich and caveolae) and outside rafts (GEM).

The fatty-acid chains of lipids within the rafts tend to be extended and so more tightly packed, creating domains with higher order. It is therefore thought that rafts exist in a separate ordered phase that floats in a sea of poorly ordered lipids. Glycosphingolipids, and other lipids with long, straight acyl chains are preferentially incorporated into the rafts.

Raft type	Constituent	Properties	Function	References
Caveolae	Cholesterol, glycoshingolipid, Arachidonic acid, Plasmenylethanolamine, Caveolin1 and 2, hetero- trimeric G-proteins and monomeric G-proteins, EGF & PDGF receptors, Fyn, GPI-linked enzymes, integrins. Flotillin	Invaginations of membranes rich in signalling molecules	Presumed to be signalling centres and perhaps regions of cholesterol import	Pike et al, 2002; Wary et al, 1998; Huang et al, 1999; Rothberg et al, 1992
Glycosphingolipid enriched	Cholesterol, glycoshingolipid, low in PI and other anionic phospholipids	Detergent resistant membranes.	Signalling?
PIP₂ enriched	PIP₂, MARKS, CAP, GAP-43		Signalling, Structural.	Laux et al, 2000; Rozelle et al, 2000.
Table 1

Caveolae are similar in composition to GEMs that lack caveolae and in fact cells that lack caveolin-1 do not have morphologically identifiable caveolae but instead have extra GEM. These cells can then be transfected with caveolin-1 cDNA and the caveolae then appear. This suggests that GEM are merely caveolae without caveolin-1. Caveolin-1 is a 21kDa integral membrane protein that binds cholesterol (Maruta et al, 1995). In cells lacking caveolin-1, caveolin-2 is synthesised but remains in the Golgi. Caveolin 1 and 2 colocalise when expressed in the same cells and they may form hetero-dimers (Scherer et al, 1997). Caveolin-3 is expressed in muscle where it forms muscle-type caveolae. Caveolin-3 is involved in certain types of muscular dystrophy (Galbiati et al, ). A slightly confusing finding is that caveolae are the reported site of integrin signalling (). It is difficult to imagine integrins being available in the depths of membrane invaginations for binding extra-cellular ligands.

The function of rafts
Many functions have been attributed to rafts, from cholesterol transport, endocytosis and signal transduction. The later is almost certainly the case. It has been suggested that the primary function of caveolae was in constitutive endocytic trafficking but recent data show that this is not the case, instead caveolae are very stable regions of membranes that are not involved in endocytosis (Thompsen et al, 2002).

Rafts and the Cytoskeleton
Many actin binding proteins are known to bind to polyphosphoinositides and to be regulated by them (see PI and ABPs), by a series of protein domains such as PH, PX and ENTH (see Domains). It is consequently scarcely surprising that some ABPs are suggested to link the actin cytoskeleton and PIP₂-enriched rafts. One of these is gelsolin, a Ca²⁺, pH and polyphosphoinositide regulated actin capping and severing protein (see Gelsolin Family), that partitions into rafts isolated biochemically from brain (Fanatsu et al, 2000).
GEMs too are suggested to link to the actin cytoskeleton through ABPs particularly ERM proteins through EBP50, a protein that binds members of the ERM proteins through the ERM C-terminus (Brdickova et al, 2001).

Deborah Brown's Lab

Kai Simon's Lab

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