According to spatial relationship of protein domains to the lipid bilayer, plasma membrane proteins fall into six categories: Type 1 and 2, which have only one transmembrane helix, the amino terminal domain is outside the cell in type 1 proteins and inside in type 2; Type 3, which have multiple transmembrane helices in a single polypeptide; Type 4, which have several transmembrane domains from different polypeptide chains to form a channel through the membrane; Type 5, which are held to the bilayer primarily by covalently linked lipid; and Type 6, which have both transmembrane helices and lip