CD5 is a T-cell-specific antigen which binds to the B-cell antigen CD72 and acts as a coreceptor in the stimulation of T-cell growth. CD5 associates with the T-cell receptor g chain (TcR()/CD3 complex and is rapidly phosphosphorylated on tyrosine residues as a result of TcRg/CD3 ligation. However, despite this, the mechanism by which CD5 generates intracellular signals is unclear. In this study, we demonstrate that CD5 is coupled to the protein-tyrosine kinase p561ck and can act as a substrate for p56Ick. Coexpression of CD5 with p56"ck in the baculovirus expression system resulted in the phosphorylation of CD5 on tyrosine residues.Further, anti-CD5 and anti-p56"k coprecipitated each other in a variety of detergents, and Triton X-100. Anti-CD5 also precipitated the kinase from various T cells irrespective of the expression of TcRg/CD3 or CD4. No binding between p590n(T) and CD5 was detected in T cells. The binding of p56Ick to CD5 induced a 10-to 15-fold increase in p561k catalytic activity, as measured by in vitro kinase analysis. In vivo labelling with 32Pi also showed a four-to fivefold increase in Y-394 occupancy in p56Ick when associated with CD5. The use of glutathione S-transferase-Lck fusion proteins in precipitation analysis showed that the SH2 domain of p56Ick could recognize CD5 as expressed in the baculovirus expression system. CD5 interaction with p561ck represents a novel variant of a receptor-kinase complex in which receptor can also serve as substrate.The CD5-p56"ck interaction is likely to play roles in T-cell signalling and T-B collaboration.The pan-T-cell marker CD5/Ly-1 antigen is a 69-kDa monomeric antigen that belongs to a family of receptors typified by the scavenger receptor cysteine-rich family of extracellular domain-like structures (19,22,38). This family includes the type I macrophage scavenger receptor, the human complement factor 1, the sea urchin speract receptor, and the lymphoid antigen CD6 (3, 25). Although highly conserved, members of this receptor family have been reported to bind ligands as diverse as speract peptides and the B-cell antigen CD72 (25). As in the case of CD2, CD4, CD6, and CD28, CD5 has been hypothesized to act as a second signal in the activation pathway of T cells. In this sense, CD5 can provide costimulatory signals in the proliferation of T cells (1,10,28,51). Costimulation increases intracellular Ca2' and cyclic GMP levels (27), interleukin 2 (IL-2) secretion, and interleukin 2 receptor (IL-2R) expression (10, 23). Other monoclonal antibodies (MAbs) to CD5 have been reported to stimulate T-cell growth directly or in conjunction with CD28 (10, 28, 63). CD5 has been reported to bind to the B-cell antigen CD72 (31, 59), a finding consistent with the observation that CD5 augments T-cell help for B-cell immunoglobulin production (55). Y-XX-P submotif in CD5. Intriguingly, the first tyrosine residue is surrounded by residues with homology to the autophosphorylation site of Src family members (DNEY) (22). The receptor therefore appears to be well constructed to ...