Classifying tetraspanins: A universal system for numbering residues and a proposal for naming structural motifs and subfamilies.

All tetraspanins have four transmembrane domains (TMs). The large extracellular loop (LEL) that connects the third and fourth TMs contains multiple secondary structures together with the family's signature Cys-Cys-Gly motif. These intriguing membrane proteins are involved in diverse and incompletely understood cellular processes including cell adhesion, tissue differentiation, immune cell maturation and host-parasite interactions.