Tcells assemble different rafts at the front and rear of the cell as they gear up to move. This is the conclusion of Santos Mañes (Universidad Autónoma de Madrid, Spain) and colleagues, who believe that the distinct rafts help T cells to do two very different things at the front and back of the cell.
Mañes concentrated on two gangliosides called GM1 and GM3—both markers for rafts and, in fibroblasts, both located at the leading edge of the moving cell. In T cells, however, Mañes found that GM3 was at the front of the cell, whereas GM1 was at the back.
The different segregation of the gangliosides and associated proteins makes sense, based on the biology of the two cell types. Fibroblasts grip tightly to their substrate as they crawl forward, and therefore have most of their cell–cell adhesion proteins at the front of the cell. The back of a fibroblast is essentially a passive tail.
By contrast, T cells skim over the surface, and have little need for cell–cell adhesion at their front end. The back of a T cell, the so-called uropod, recruits bystander cells using intercellular adhesion molecules (ICAMs). It is this need to act as a bipolar sensor that distinguishes T cells.
The differential localization of GM1 and GM3 requires an intact actin cytoskeleton. How the polarization is initiated, or how the different raft proteins are sorted from one another, is not yet known. At the front of the cell, activated chemokine receptors are the best candidates for initiating polarization. ▪