  Lipid rafts are heterogeneous dynamic lipid domains of the cell membranes
that are involved in several biological processes, like protein and lipids
specific transport and signaling. Our understanding of lipid raft formation is
still limited, due to the transient and elusive nature of these domains {\it in
vivo}, in contrast to the stable phase-separated domains observed in artificial
membranes. Inspired by experimental findings highlighting the relevance of
transmembrane proteins for lipid rafts, we investigate lipid domain nucleation
by coarse-grained molecular dynamics and Ising model simulations. We find that
the presence of a transmembrane protein can trigger lipid domain nucleation in
a flat membrane from an otherwise mixed lipid phase. Furthermore, we study the
role of the lipid domain in the diffusion of the protein showing that its
mobility is hindered by the presence of the raft. The results of our
coarse-grained molecular dynamics and Ising model simulations thus coherently
support the important role played by transmembrane proteins in lipid domain
formation and stability.
