Expression cloning of the early activation antigen CD69, a type II integral membrane protein with a C-type lectin domain.

CD69 is a very early activation Ag of T lymphocytes. It is a cell surface glycoprotein that can only be detected after stimulation of lymphocytes. Despite extensive studies on its biologic activities, little is known about its molecular function. To investigate the latter in more detail, we have cloned a cDNA encoding CD69 on the basis of its expression in COS cells. The nucleotide sequence of clone CD69.13 is 1676 bp in length and contains a single open reading frame of 600 bp encoding a protein of 199 amino acids. The predicted molecular mass of 22,559 Da could be confirmed by in vitro translation. The protein contains a hydrophobic transmembrane region between amino acids 41 and 61 but no N-terminal signal peptide, which suggests that it is a type II membrane protein. It has one potential N-glycosylation site at amino acid 166. Two glycosylated forms of 26 to 28 kDa and 32 to 34 kDa were detected both in transfected COS cells and in in vitro translation in the presence of canine microsomes. Proteinase K degradation of the N-terminal part after in vitro protein synthesis supports the view of CD69 being a type II integral membrane protein with the N-terminal 40 amino acids in the cytoplasm, a transmembrane domain of 21 amino acids, and C-terminal 138 amino acids as the extracellular domain. Homology searches revealed sequence similarity with members of a supergene family of type II integral membrane proteins with a C-type lectin domain, indicating that CD69 is involved in signal transduction.