  The recent discovery of a lateral organization in cell membranes due to small
structures called 'rafts' has motivated a lot of biological and
physico-chemical studies. A new experiment on a model system has shown a
spectacular budding process with the expulsion of one or two rafts when one
introduces proteins on the membrane. In this paper, we give a physical
interpretation of the budding of the raft phase. An approach based on the
energy of the system including the presence of proteins is used to derive a
shape equation and to study possible instabilities. This model shows two
different situations which are strongly dependent on the nature of the
proteins: a regime of easy budding when the proteins are strongly coupled to
the membrane and a regime of difficult budding.
