ReviewTrafficking and function of the tetraspanin CD63
Section snippets
The tetraspanin family
The tetraspanin family was first recognized in 1990, when sequences from Cluster of Differentiation (CD) 37 and CD81 were compared to the tumour-associated gene CD63 [1]. This revealed sequence homology and a conserved predicted structure of 4 hydrophobic transmembrane domains, a small and a large extracellular loop and 2 short intracellular amino- and carboxyl tails. A typical tetraspanin consists of 200–300 amino acids and contains 4–8 conserved extracellular cysteines of which 2 are present
Tetraspanin trafficking
Like most transmembrane proteins, tetraspanins are synthesized in the endoplasmic reticulum (ER). The transmembrane regions of tetraspanins are important for ER exit, since deletion of one or more transmembrane domains of CD9, CD151, CD82 and uroplakin Ib resulted in ER retention, even when their extracellular domain was properly folded [7], [8], [9], [10]. Many tetraspanins are palmitoylated, which occurs in the Golgi complex [11]. After palmitoylation, tetraspanins often form homodimers,
CD63
The CD63 gene is located on human chromosome 12q13 and was the first characterized tetraspanin. Originally, however, CD63 was discovered as a protein present on the cell surface of activated blood platelets, known as platelet glycoprotein 40 (Pltgp40) [30] and in early stage human melanoma cells, where it was known as melanoma antigen 491 (ME491) [31], [32]. CD63, being a tetraspanin, interacts with many different proteins either directly or indirectly. Interaction partners include integrins
Trafficking of CD63
CD63 is a ubiquitously expressed protein that is localized within the endosomal system and at the cell surface (Fig. 1). In most cells the major pool of CD63 resides in late endosomes/MVBs and lysosomes, which is why it is also referred to as a lysosomal membrane protein. Lysosomal membrane proteins that exit the TGN can travel to lysosomes via either a direct TGN-to-endosome pathway or an indirect route, involving passage over the plasma membrane and subsequent endocytosis (Fig. 1). In
CD63 in caveolae
An alternative pathway for clathrin-mediated endocytosis is provided by caveolae. These typical flask-like shaped 50–100 nm diameter plasma membrane invaginations mediate endocytosis in a dynamin-dependent, but clathrin-independent manner [55]. The best characterized entry via caveolae is for Simian virus 40 (SV40) [56]. Furthermore, Echovirus (EV1) is using α2β1-integrin to gain entry into the cell via caveolae [57]. Other proteins, such as cholera toxin and albumin can also be endocytosed via
AP-3 dependent transport of CD63
Transport of CD63 from early endosomes to late endosomes and lysosomes may involve at least two different pathways: 1. incorporation into ILVs (see previous paragraph) or 2. the AP-3 pathway. In previous immuno-electron microscopy studies, we localized AP-3 to buds emerging from tubular endosomal membranes (Fig. 1) that contain proteins for distinct destinations in the cell, i.e. TGN, plasma membrane and lysosomes [51], [59] and which we called ‘tubular sorting endosomes’ (TSE). We postulated
Cell surface expression of CD63
An increasing number of studies indicate that in certain conditions the cell surface expression of CD63 is tightly regulated, which likely is important for its functioning within TEMs. Here we discuss syntenin-1 and L6-antigen, two proteins that mediate cell surface levels of CD63.
Possible functions of CD63 in intracellular trafficking
Although the intracellular function of CD63 remains to be established, a number of studies performed in different cell types implicate a role for CD63 in intracellular transport of other proteins.
CD63 and cancer
CD63 was first discovered as an abundantly expressed surface antigen in early stage melanoma cells [31]. However, during further malignant melanoma progression CD63 expression is reduced, while cells become more invasive, suggesting a negative correlation between cell surface expression of CD63 and tumour invasiveness. Indeed, silencing of CD63 by RNAi in melanoma cells results in increased cell motility and matrix-degrading ability [67], whereas in a reversed approach, in which a CD63-negative
Concluding remarks
In this review, we have summarized current insights in CD63 functioning and trafficking. Although AP-2-mediated uptake of CD63 via clathrin coated pits is well established, recent studies suggest the existence of alternative pathways of CD63 trafficking from the cell surface to endosomes/lysosomes. Syntenin-1 might divert CD63 from the clathrin-mediated endocytosis pathway to another pathway that has yet to be described in detail, while our own data indicate that CD63 may be internalized via
Acknowledgments
We thank Hans Geuze, Harry Heijnen and Vincent Schoonderwoert for critical reading of this manuscript, Andrew Peden for kindly providing us with the Mocha CD63 cells, Viola Oorschot for technical support and Marc van Peski and René Scriwanek for preparation of the figures. We also thank Harry Heijnen and Vincent Schoonderwoert for sharing non-published data with us. The anti-CD63 antibody was purchased from the Developmental Studies Hybridoma Bank, the anti-caveolin-1 antibody from Transduction
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