Tetraspanin, extracellular domain or large extracellular loop (LEL), uroplakin_I_like family. Tetraspanins are trans-membrane proteins with 4 trans-membrane segments. Both the N- and C-termini lie on the intracellular side of the membrane. This alignment model spans the extracellular domain between the 3rd and 4th trans-membrane segment. Tetraspanins are involved in diverse processes and their various functions may relate to their ability to act as molecular facilitators. Tetraspanins associate laterally with one another and cluster dynamically with numerous parnter domains in membrane microdomains, forming a network of multimolecular complexes, the "tetraspanin web". Uroplakin Ia and Ib are components of the 16nm protein particles, which are packed hexagonally to form 2D crystals of asymmetric unit membranes, and cover the apical surface of mammalian urothelium, contributing to the urinay bladder's permeability barrier function. Uroplakins Ia and Ib are maturation facilitators. They trigger conformational changes in their single-transmembrane-domain binding partner proteins uroplakin II and IIIa, which in turn may lead to ER-exit, stabilization, and cell-surface expression.
Jiyao W et al.(2023). "The conserved domain database in 2023.",