  Heterogeneities in the cell membrane due to coexisting lipid phases have been
conjectured to play a major functional role in cell signaling and membrane
trafficking. Thereby the material properties of multiphase systems, such as the
line tension and the bending moduli, are crucially involved in the kinetics and
the asymptotic behavior of phase separation. In this Letter we present a
combined analytical and experimental approach to determine the properties of
phase-separated vesicle systems. First we develop an analytical model for the
vesicle shape of weakly budded biphasic vesicles. Subsequently experimental
data on vesicle shape and membrane fluctuations are taken and compared to the
model. The combined approach allows for a reproducible and reliable
determination of the physical parameters of complex vesicle systems. The
parameters obtained set limits for the size and stability of nanodomains in the
plasma membrane of living cells.
