Ordered adhesions

The plasma membrane of an adherent cell is most highly ordered where it binds to the extracellular matrix, report Gaus et al. on page 725. This suggests that tiny raft-like domains may form into larger structures around these attachment sites.

Rafts have been controversial because much of the raft literature is based on in vitro isolation of detergent-resistant membranes, dubbed rafts. What this means in vivo has been less clear. Gaus et al. use an alternative, in vivo mechanism to measure the degree of order in a membrane: the fluorescent dye Laurdan. Its emission spectrum shifts to a longer wavelength when the dye is in less-ordered membranes that allow greater penetration of water molecules.

Caveolin-rich regions of endothelial cell membranes were highly ordered relative to the average value of the membranes as a whole, but the most highly ordered regions were at caveolin-rich focal adhesions (FAs). The caveolin at FA sites is known to be in its phosphorylated form (pYCav1), and the group showed that this phosphorylation is necessary to establish the highly ordered structure of FAs.

Detachment causes rapid internalization of some of these ordered domains. As these domains do a lot of signaling, the internalization may reduce the signaling that normally maintains attachment-dependent growth.