Transmembrane proteins form an important class of membrane proteins that permeate the entire phospholipid bilayer of a cell. They account for almost one third of the total protein content of cells and are of great importance for the functionality of the cell and the cell organelles.
A distinction is made between singlepass transmembrane proteins, which cross the membrane only once, and multipass transmembrane proteins, which cross the membrane several times.
The singlepass transmembrane proteins are called type-1 transmembrane proteins if the N-terminus is located extracellularly.
The singlepass transmembrane proteins are called type-2 transmembrane proteins when the N-terminus is intracellular.