The T-Cell Antigen CD5 Acts as a Receptor and Substrate for the Protein-Tyrosine Kinase p56^lck

Abstract

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 ζ chain (TcRζ)/CD3 complex and is rapidly phosphosphorylated on tyrosine residues as a result of TcRζ/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 p56^lck and can act as a substrate for p56^lck. Coexpression of CD5 with p56^lck in the baculovirus expression system resulted in the phosphorylation of CD5 on tyrosine residues. Further, anti-CD5 and anti-p56^lck coprecipitated each other in a variety of detergents, including Nonidet P-40 and Triton X-100. Anti-CD5 also precipitated the kinase from various T cells irrespective of the expression of TcRζ/CD3 or CD4. No binding between p59^fyn(T) and CD5 was detected in T cells. The binding of p56^lck to CD5 induced a 10- to 15-fold increase in p56^lck catalytic activity, as measured by in vitro kinase analysis. In vivo labelling with ³²P_i also showed a four- to fivefold increase in Y-394 occupancy in p56^lck when associated with CD5. The use of glutathione S-transferase-Lck fusion proteins in precipitation analysis showed that the SH2 domain of p56^lck could recognize CD5 as expressed in the baculovirus expression system. CD5 interaction with p56^lck represents a novel variant of a receptor-kinase complex in which receptor can also serve as substrate. The CD5-p56^lck interaction is likely to play roles in T-cell signalling and T-B collaboration.