class of proteins
Membrane proteins perform a variety of functions vital to the survival of organisms:
- Membrane receptor proteins relay signals between the cell's internal and external environments.
- Membrane transport proteins move molecules and ions across the membrane.
- Membrane enzymes have many activities.
- Cell adhesion molecules allow cells to identify each other and interact. For example, proteins involved in immune response.
- Krogh A. et al 2001. Predicting transmembrane protein topology with a hidden markov model: application to complete genomes. Journal of Molecular Biology 305 (3): 567–580. 
- Overington J.P; Al-Lazikani B & Hopkins A.L. (2006). "How many drug targets are there?". Nat Rev Drug Discov. 5 (12): 993–6. doi:10.1038/nrd2199. PMID 17139284.
- Almén M. et al 2009. Mapping the human membrane proteome: a majority of the human membrane proteins can be classified according to function and evolutionary origin. BMC Biology 7: 50. 
- General principles of membrane protein folding and stability from Stephen White laboratory
- Orientations of proteins in membranes (OPM) database 3D structures of integral and amphitropic membrane proteins