ies with chimeric molecules demonstrated that se- 
quences contained within the transmembrane do- 
mains of the TCR chains are responsible for the 
association of the TCR subunits. Dr. Weiss and his 
colleagues took advantage of this information to de- 
velop a strategy to study the function of domains of 
individual TCR chains. A chimeric molecule in 
which the cytoplasmic domain of f was fused to the 
extracellular and transmembrane domains of CDS 
was constructed and could be expressed in the ab- 
sence of other TCR chains. This permitted the analy- 
sis of the function of the cytoplasmic domain of f 
independently of other TCR chains. Remarkably, 
stimulation of CD8/f induced all of the early and 
late events associated with stimulation of the intact 
oligomeric TCR. 
More recent studies have identified a 22-residue 
sequence motif, based on conservatively placed ty- 
rosine and leucine residues, that is sufficient to cou- 
ple chimeric receptors to the TCR-regulated signal 
transduction events. This motif is triplicated within 
f and is contained as a single copy in all three CD3 
chains as well as in the non-ligand-binding chains of 
the B cell antigen receptor and the mast cell IgE Fc 
receptor. Thus it appears that these oligomeric re- 
ceptors expressed on distinct cell types of the he- 
matopoietic lineage utilize a similar structural mo- 
tif to couple to intracellular signal transduction 
components. 
Stimulation of the TCR activates a protein- 
tyrosine kinase (PTK) not intrinsic to the structure 
of the receptor. Activation of this PTK induces the 
tyrosine phosphorylation of several proteins, in- 
cluding the TCR f chain, phospholipase C (PLC)- 
7I, mitogen-activated protein kinase-2, and the 
proto-oncogene Vav. Dr. Weiss and his colleagues 
used signal transduction mutant cell lines to show 
that phosphorylation of PLC-7 1 is associated with its 
activation. (This work was supported by a grant 
from the National Institutes of Health.) 
Since f can link the TCR to intracellular signal 
transduction components, experiments during the 
past year were directed toward identifying mole- 
cules that interact with the cytoplasmic domain of f . 
A 70-kDa tyrosine phosphoprotein rapidly asso- 
ciates with f following TCR stimulation. This pro- 
tein, termed ZAP-70 (for ^associated protein), asso- 
ciates with f via sequences contained within the f 
activation motif, and this correlates with increased 
PTK activity in f immunoprecipitates from stimu- 
lated cells. Preliminary sequence and expression 
analyses reveal that ZAP-70 is a PTK. It is likely to be 
involved in TCR-mediated signal transduction. 
Recent studies of J.CaMl, a previously isolated 
mutant cell line with a defect in TCR-mediated sig- 
nal transduction, implicate another PTK in the path- 
way. (This work was supported by a grant from the 
National Institutes of Health.) Stimulation of the 
TCR on J.CaMl fails to induce PTK or PLC activities. 
This cell has been found to be deficient in Lck PTK 
function as a result of an abnormally spliced Lck 
transcript. TCR signaling function was restored by 
transfection with Lck cDNA. Thus these studies im- 
plicate two distinct PTKs, ZAP-70 and Lck, in TCR 
signal transduction. It will be interesting to deter- 
mine if these PTKs interact during the transduction 
process. 
CD45 Tyrosine Phosphatase Regulates 
TCR Signal Transduction 
CD45 is a transmembrane tyrosine phosphatase 
expressed on most hematopoietic cells. Various iso- 
forms of CD45, the products of alternative splicing 
of the same gene, are expressed in a tissue and acti- 
vation manner. Mutant cell lines deficient in CD45 
expression have defects in TCR-mediated induction 
of PTK and PLC activities. The signaling functions of 
these mutants have now been reconstituted with ei- 
ther the 2 2 0 - or 1 8 0 -kDa isoforms of CD4 5 . Work is 
in progress to assess the role of the extracellular 
domain in regulating CD45 function. 
Of considerable interest is the identification of 
the relevant intracellular targets of CD45. Potential 
targets of CD45 include members of the Src family 
of PTKs. These PTKs each have a negative regulatory 
tyrosine phosphorylation site near their carboxyl 
termini. Thus the signaling defect in CD45- 
deficient cells might involve a Src PTK family 
member critical in TCR-mediated signal transduc- 
tion that is inactive because of hyperphosphoryla- 
tion at its negative regulatory site. Indeed, prelimi- 
nary assays of Lck isolated from CD4 5 -deficient cells 
reveal it to be hyperphosphorylated at this site. Thus 
a PTK shown to be required for TCR-mediated signal 
transduction in the J.CaMl mutant is also negatively 
regulated in CD45-deficient cells. Future experi- 
ments are aimed at determining whether the status 
of Lck phosphorylation is the sole explanation 
for the signal transduction deficiency in CD45- 
deficient cells. 
CD28 Regulation of Lymphokine 
Gene Expression 
Stimulation of the TCR alone does not suffice to 
initiate lymphokine gene expression or T cell prolif- 
eration. Stimulation of other receptors, such as 
CD28, provides a co-stimulatory function. CD28 
binds to B7, a molecule expressed on potent APCs 
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