Crystal structure of human CD69: A C-type lectin-like activation marker ofhematopoietic cells

CD69 is a widely expressed type II transmembrane glycoprotein related to th e C-type animal lectins that exhibits regulated expression on a variety of cells of the hematopoietic lineage, including neutrophils, monocytes, T cel ls, B cells, natural killer (NK) cells, and platelets. Activation of T lymp hocytes results in the induced expression of CD69 at the cell surface. In a ddition, cross-linking of CD69 by specific antibodies leads to the activati on of cells bearing this receptor and to the induction of effector function s. However, the physiological ligand of CD69 is unknown. We report here the X-ray crystal structure of the extracellular C-type lectin-like domain (CT LD) of human CD69 at 2.27 Angstrom resolution. Recombinant CD69 was express ed in bacterial inclusion bodies and folded in vitro. The protein, which ex ists as a disulfide-linked homodimer on the cell surface, crystallizes as a symmetrical dimer, similar to those formed by the related NK cell receptor s Ly49A and CD94. The structure reveals conservation of the C-type lectin-l ike fold, including preservation of the two alpha -helical regions found in Ly49A and mannose-binding protein (MBP). However, only one of the nine res idues coordinated to Ca2+ in MBP is conserved in CD69 and no bound Ca2+ is evident in the crystal structure. Surprisingly, electron density suggestive of a puckered six-membered ring was discovered at a site structurally anal ogous to the ligand-binding sites of MBP and Ly49A. This sugar-like density may represent, or mimic, part of the natural ligand recognized by CD69.