DIMERIC ASSOCIATION AND SEGMENTAL VARIABILITY IN THE STRUCTURE OF HUMAN CD4

CD4 is a co-receptor in the cellular immune response, It increases the avidity of association between a T cell and an antigen-presenting cel l by interacting with non-polymorphic portions of the complex between class II major histocompatibility complex (MHC) and T-cell receptor (T CR) molecules, and it contributes directly to signal transduction thro ugh its cytoplasmic association with the lymphocyte kinase Lck (ref. 1 ). CD4 also serves as the high-affinity receptor for cellular attachme nt and entry of the human immunodeficiency virus (HIV)(2). The extrace llular portion of CD4 comprises four immunoglobulin-like domains (D1-D 4), This part of human CD4 (residues 1-369) has been characterized as a recombinant soluble protein (sCD4)(3,4), and crystal structures have been described for the human D1D2 fragment(5,6) and for the rat D3D4 fragment(7). We have now determined the structures of intact sCD4 in t hree crystal lattices. These structures have a hinge-like variability at the D1D2 to D3D4 junction that might be important in immune recogni tion and HIV fusion, and a common dimeric association through D4 domai ns. Dynamic light scattering measurements and chemical crosslinking of sCD4 corroborate dimerization at high protein concentration. We sugge st that such dimers may have relevance as mediators of signal transduc tion in T cells.