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Covalent binding of guanine nucleotides to the CD3-gamma chain of the T cell receptor/CD3 complex.

In a search for proteins involved in signal transduction through the T cell receptor (TcR/CD3 complex), a recently developed highly efficient method for labeling of nucleotide binding proteins in permeabilized cells was applied. Here, we report that human CD3-gamma could be labeled by periodate-oxidized [alpha-32P] GTP (GTPoxi). In contrast to GTPoxi labeling of CD3-zeta (Peter, M. E., Hall, C., Rühlmann, A., Sancho, J. and Terhorst, C., EMBO J. 1992. 11:933), GTP-specific labeling of CD3-gamma reached a maximum when nucleotides were added 60 min prior to the cross-linking reaction. As CD3-gamma did not contain a known consensus sequence for nucleotide binding and since labeling kinetics of CD3-gamma coincided with those of cytosolic GTP-binding proteins, labeling may have been caused by a GTP-binding protein. This putative protein was not T cell specific because labeling of CD3-gamma could also be achieved when expressed in the endoplasmic reticulum of Chinese hamster ovary (CHO) cells. In CHO cells, labeling by GTPoxi took place only when CD3-gamma was associated with CD3-epsilon, whereas labeling could not be established upon association of CD3-gamma with CD3-delta or TcR alpha. The observation that CD3-gamma was labeled without leaving the endoplasmic reticulum led to the hypothesis that the association of CD3-gamma with a GTP-binding protein might be involved in an early step of the TcR/CD3 complex formation or transport.