Transmembrane

A transmembrane protein is a protein which is associated with the lipid bilayer in the membrane. The protein runs right through the membrane with both cytsolic and extracellular domains. Transmembrane proteins are amphipathic; the hydrophobic regions interact with lipid tails inside the lipid bilayer and the hydrophillic regions are exposed.

Reference

Alberts,B., Johnson,A., Lewis,J., Raff,M., Roberts,K. and Walter,P. (2008) Molecular Biology of the Cell, 5th Edition, 630