The cytoskeleton-associated TCR ζ 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 ζ (cska-ζ) chain, while the other is detergent soluble and not linked to the cytoskeleton. The cska-ζ 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-ζ) 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 ζ form. We now demonstrate that the posphorylation of cska-ζ does not require the activity of p56^lck. We also show that although Lck does not phosphorylate cska-ζ in vivo, it retains the capacity to phosphorylate cska-ζ in vitro. Moreover, differences in ζ-associated kinase activity were detected for non-cska-ζ and cska-ζ. Our results indicating that different kinases phosphorylate the two ζ forms are consistent with a growing consensus that each TCR form may regulate distinct cellular functions.