TI  - The T-cell antigen  <prot>CD5</prot>  acts as a receptor and substrate for the
      protein-tyrosine kinase  <prot>p56lck</prot> .
PG  - 2862-70
AB  -  <prot>CD5</prot>  is a T-cell-specific antigen which binds to the B-cell antigen  <prot>CD72</prot> 
      and acts as a coreceptor in the stimulation of T-cell growth.  <prot>CD5</prot> 
      associates with the  <prot>T-cell receptor zeta chain</prot> (<prot>TcR zeta</prot>)/<prot>CD3</prot>  complex and
      is rapidly phosphosphorylated on tyrosine residues as a result of  <prot>TcR
      zeta</prot> / <prot>CD3</prot>  ligation. However, despite this, the mechanism by which <prot>CD5</prot>
      generates intracellular signals is unclear. In this study, we demonstrate
      that   <prot>CD5</prot>   is coupled to the  protein-tyrosine kinase <prot>p56lck</prot>  and can act as a
      substrate for  <prot>p56lck</prot> . Coexpression of <prot>CD5</prot> with <prot>p56lck</prot> in the baculovirus
      expression system resulted in the phosphorylation of <prot>CD5</prot> on tyrosine
      residues. Further, anti-<prot>CD5</prot> and anti-<prot>p56lck</prot> coprecipitated each other in a
      variety of detergents, including Nonidet P-40 and Triton X-100. Anti-<prot>CD5</prot>
      also precipitated the kinase from various T cells irrespective of the
      expression of <prot>TcR zeta</prot>/<prot>CD3</prot> or <prot>CD4</prot>. No binding between <prot>p59fyn</prot>(T) and <prot>CD5</prot>
      was detected in T cells. The binding of  <prot>p56lck</prot>  to  <prot>CD5</prot>  induced a 10- to
      15-fold increase in <prot>p56lck</prot> 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  <prot>p56lck</prot>  when associated with  <prot>CD5</prot> .
      The use of glutathione S-transferase-Lck fusion proteins in precipitation
      analysis showed that the SH2 domain of <prot>p56lck</prot> could recognize <prot>CD5</prot> as
      expressed in the baculovirus expression system. <9:1> <prot>CD5</prot> </9:1> interaction with
      <9:2> <prot>p56lck</prot> </9:2> represents a novel variant of a receptor-kinase complex in which
      receptor can also serve as substrate.(ABSTRACT TRUNCATED AT 250 WORDS)
AD  - Division of Tumor Immunology, Dana-Farber Cancer Institute, Boston,
