Figure 1.

Specific activation of Lck and Fyn tyrosine kinases by CD2 and CD28 peptides in vitro. (A) Sequences of peptides based on the cytoplasmic domain sequences of CD2, CD28, and CD3∈ are shown using the one-letter amino acid code. Phenylalanine was substituted for tyrosine in the CD3∈ peptide to prevent phosphorylation of this peptide in vitro. (B) Activation of Lck in vitro by a CD28 peptide. Purified Lck was preincubated with each of the peptides listed in A for 20 min on ice, and kinase activity was measured using a Src substrate peptide and [γ-³²P]ATP as described by Moarefi et al. 8. Bar graphs represent fold activation of kinases incubated with peptides as compared with the activity of kinase incubated with buffer and substrate alone. (C) Activation of Fyn in vitro by a CD2 peptide. Kinase reactions were performed as in B except that purified Fyn was used instead of Lck. All data are representative of at least three independent trials.