BioPHYSICAL chemistry

the raft. In these models, signaling could be initiated through the raft (see
Figure 4.8). Many of these proteins are active only after dimerization or
association with other proteins. The enrichment of specific proteins in the
raft may facilitate this process. Two proteins within the raft may become
activated upon association with other proteins. Alternatively, different rafts
may become clustered, leading to dimerization of the protein components.
The interactions that drive raft assembly are dynamical and reversible,
providing a mechanism for turning off the activity.

Determination of micelle formation using surface tension

The formation of these lipid and detergent structures can be measured by
a number of physical properties. As an example, the formation of micelles
can be determined by measuring the surface tension of the solution. Liquid
drops will generally adopt a shape that minimizes the exposed surface area,

CHAPTER 4 PHASE DIAGRAMS AND MIXTURES 79

GP1

a

b

GP1GP1

GP1

c

Dimerization Clustering agent

Dimerization

Extracellular

(a)

(b)

Signaling Signaling

Antibodies,

ligands

Antibodies,

ligands

Dimerization

Activation in a raft

Clustering of rafts triggers signaling

Altered partitioning

Extracellular

(antibodies, ligands)

Cytosolic

(cytoskeletal elements,

adaptors, scaffolds)

Membrane protein

(for example, LAT)

Figure 4.8Models for the involvement of lipid rafts in cellular functions,
with signaling initiated in (a) single rafts or (b) clustered rafts. Modified
from Simons and Toomre (2000).