ULBP-1, -2, and -3 were originally identified as ligands for the human cytomegalovirus glycoprotein UL16 and designated UL16-binding proteins (ULBP). These proteins are distantly related to major histocompatibility class I (MHC I) molecules, possessing the alpha 1 and alpha 2 Ig-like domains, but lacking the alpha 3 domain. Unlike MHC Class I, they have no capacity to bind peptide or interact with beta2-microglobulin. ULBP-1, -2, and -3 are known to bind to human NKG2D, an activating receptor expressed on NK cells, NKT cells, gamma delta T cells, and CD8+ alpha beta T cells.
MULT1 (mouse UL16-binding protein-like Transcript 1) is a 53 kDa, MHC Class I-like molecule that belongs to the mouse family of NKG2D ligands. It is a type I transmembrane glycoprotein that is synthesized as a 334 amino acid (aa) precursor. It contains a 25 aa signal sequence, a 186 aa extracellular region, a 19 aa transmembrane segment and a 104 aa cytoplasmic tail. The extracellular region contains an alpha-1 and alpha-2 like domain with two intrachain disulfide bonds.