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 surf ace glycoprotein that can only be detected after stimulation of lympho cytes. 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 expre ssion in COS cells. The nucleotide sequence of clone CD69.1 3 is 1676 bp in length and contains a single open reading frame of 600 bp encodi ng a protein of 199 amino acids. The predicted molecular mass of 22,55 9 Da could be confirmed by in vitro translation. The protein contains a hydrophobic transmembrane region between amino acids 41 and 61 but n o N-terminal signal peptide, which suggests that it is a type II membr ane protein. It has one potential N-glycosylation site at amino acid 1 66. Two glycosylated forms of 26 to 28 kDa and 32 to 34 kDa were detec ted both in transfected COS cells and in in vitro translation in the p resence of canine microsomes. Proteinase K degradation of the N-termin al part after in vitro protein synthesis supports the view of CD69 bei ng a type II integral membrane protein with the N-terminal 40 amino ac ids in the cytoplasm, a transmembrane domain of 21 amino acids, and C- terminal 138 amino acids as the extracellular domain. Homology searche s revealed sequence similarity with members of a supergene family of t ype II integral membrane proteins with a C-type lectin domain, indicat ing that CD69 is involved in signal transduction.