Plasma membrane microdomains: organization, function and trafficking

Abstract

The plasma membrane consists of a mosaic of functional microdomains facilitating a variety of physiological processes associated with the cell surface. In most cells, the majority of the cell surface is morphologically featureless, leading to difficulties in characterizing its organization and microdomain composition. The reliance on indirect and perturbing techniques has led to vigorous debate concerning the nature and even existence of some microdomains. Recently, increasing technical sophistication has been applied to study cell surface compartmentalization providing evidence for small, short-lived clusters that may be much less than 50 nm in diameter. Lipid rafts and caveolae are cholesterol-dependent, highly ordered microdomains that have received most attention in recent years, yet their precise roles in regulating functions such as cell signalling remain to be determined. Endocytosis of lipid rafts/caveolae follows a clathrin-independent route to both early endosomes and non-classical caveosomes. The observation that a variety of cellular pathogens localize to and internalize with these microdomains provides an additional incentive to characterize the organization, dynamics and functions of these domains.

Figure 1

Plasma membrane compartmentalisation. Plasma membrane microdomains exist within a secondary macro-organisation imposed by the actin cytoskeleton and anchored transmembrane proteins. A variety of types of microdomain have been identified, however their size, organisation and distribution remain to be accurately determined. Caveolae and lipid rafts are cholesterol-dependent microdomains whose organisation is proposed to facilitate sorting of proteins and lipids that can intercalate into the highly ordered structure. Caveolin drives the formation of caveolae and is proposed to interact with a wide variety of proteins via the caveolin scaffolding domain. This Figure is reproduced in colour in Molecular Membrane Biology on-line.

Figure 2

Electron microscopic imaging of microdomains Electron microscopy of isolated plasma membrane sheets provides nanoscale resolution for characterising microdomains. Caveolae (A) possess distinctive morphology but in most cell types the majority of the cell surface is morphologically featureless (B, C). Single (B) or double gold labelling (C) of microdomain markers and proteins of interest coupled with spatial statistical analysis of clustering patterns allows the size and abundance of microdomains and the degree of protein co-localisation to be determined (Prior et al., 2003). Bars = 50nm.

Figure 3

Compartmentalised cell signalling. Ras isoform signalling is regulated by precise microlocalisation. Disruption of fibroblast lipid rafts/caveolae inhibits H-ras but not K-ras signalling. H-ras activation promotes migration from caveolae and lipid rafts to non-raft signalling microdomains that are distinct from K-ras signalling domains. eNOS signalling is regulated by caveolin; cell stimulation promotes calcium entry and eNOS/caveolin dissociation allowing eNOS activation and nitric oxide (NO) production. Many regulators and targets of eNOS/NO have been localised to caveolae/lipid rafts. This Figure is reproduced in colour in Molecular Membrane Biology on-line.

Figure 4

Endocytosis of caveolae and lipid rafts. After internalisation lipid raft and caveolae-derived vesicles fuse with caveosomes where their cargo is sorted for delivery to the endoplasmic reticulum (ER), Golgi and the classical endocytic system. The connections between caveosomes and the classical endocytic sytem are poorly defined; studies of glycosphingolipid (GSL) trafficking represent the best example of a putative link. Unlike the other pathways, pinocytosis is dynamin-independent and represents an alternative route for GPI-anchored protein endocytosis; this pathway ultimately merges with the classical endocytic system in perinuclear recycling endosomes. Abbreviations: Cholera Toxin, CTx; Transferrin, Tfn. This Figure is reproduced in colour in Molecular Membrane Biology on-line.