The cytoskeleton-associated TCR zeta chain is constitutively phosphorylated in the absence of an active p56(lck) form

The TCR recognizes peptide-MHC complexes and transmits activation signals leading to cellular responses. We have previously characterized two TCR populations expressed on the T cell surface; one is linked to the cytoskeleton via a detergent-insoluble cytoskeleton-associated zeta (cska-zeta) chain, while the other is detergent soluble and not linked to the cytoskeleton. The cska-zeta form displays unique properties: it is constitutively phosphorylated, does not undergo hyperphosphorylation upon TCR stimulation as opposed to its non-cytoskeleton-associated counterpart (non-cska-zeta) and it maintains a molecular mass of 16 kDa. It is well established that p56(lck) and possibly p59(fyn) are responsible for the generation of the 21/23-kDa phosphorylated detergent-soluble zeta form. We now demonstrate that the phosphorylation of cska-zeta does not require the activity of p56(lck). We also show that although Lck does not phosphorylate cska-zeta in vivo, it retains the capacity to phosphorylate cska-zeta in vitro. Moreover, differences in zeta-associated kinase activity were detected for non-cska-zeta and cska-zeta. Our results indicating that different kinases phosphorylate the two zeta forms are consistent with a growing consensus that each TCR form may regulate distinct cellular functions.