of phosphotyrosine proteins from the thymoma cell 
line LSTRA. Thus SHP may interact through the SH2 
domains with a specific set of tyrosine-phosphory- 
lated proteins. 
PEP is an additional intracellular PTPase ex- 
pressed by many different types of leukocytes. In 
contrast to SHP, it is revealed by in situ hybridiza- 
tion to be uniformly expressed in thymus, spleen, 
and lymph nodes. Sequence analysis of PEP dis- 
closed several putative nuclear-localizing signals. 
Furthermore, near the carboxyl terminus there are 
sequence motifs indicative of rapid degradation, 
suggesting that PEP protein expression may be 
highly regulated. 
Phosphorylation is an important regulator of tran- 
scriptional activity, yet tyrosine phosphorylation's 
role in regulating transcription is unknown. To 
analyze whether PEP is a nuclear phosphatase, 
members of Dr. Thomas's laboratory epitope-tagged 
PEP by the addition of a small sequence with a de- 
fined antibody-binding site. Transfection of the 
epitope-tagged PEP into HeLa cells revealed a nu- 
clear localization. This raises the exciting possibil- 
ity that PEP may function in regulating nuclear 
events. Further insights into the role of tyrosine 
phosphorylation in controlling nuclear events 
should follow from mutagenesis dissection of PEP 
function. 
Transmembrane PTPases Expressed 
by Lymphocytes 
CD45 is an abundant transmembrane PTPase of 
leukocytes. Dr. Thomas and his colleagues function- 
ally characterized CD45 by developing CD45- 
deficient T cell lines. This research demonstrated a 
critical role for CD45 in antigen-induced activation. 
CD45 deficiency leads to the inability to respond 
properly to T cell receptor stimuli. Thus signaling 
through the T cell receptor requires the expression 
of CD45 . However, it is possible to bypass the signal- 
ing defect by directly activating protein kinase C 
through T cell receptor stimulation. This suggests 
that CD45 functions proximal to protein kinase C 
activation. 
Analysis of tyrosine-phosphorylated proteins in 
the CD4 5 -deficient cells revealed that members of 
the Src-tyrosine kinase family are increased in tyro- 
sine phosphorylation and are therefore potential 
substrates for CD45. Src-family members are nega- 
tively regulated by tyrosine phosphorylation of a 
carboxyl-terminal site. Thus the increase in phos- 
phorylation in Src-family members in CD45- 
deficient cells may lead to decreased kinase activity. 
Three members of the Src family are expressed in 
T cells, ^56"", p59^", and p62J'^^ Examination by 
members of Dr. Thomas's laboratory has indicated 
that the kinase activities for all three are decreased 
in the CD45-deficient T cells. Furthermore, in col- 
laboration with Dr. Bart Sefton of the Salk Institute, 
analysis of the negative regulatory site demonstrated 
increased phosphorylation at this site in the 
CD4 5 -deficient cells. This indicates that CD45 
functions to regulate Src-family-member kinase ac- 
tivity by dephosphorylating the negative regulatory 
site. For T cells, this function is required for signal- 
ing through the T cell receptor. (Work on CD45- 
deficient cells is supported by grants from the Na- 
tional Institutes of Health and the Council for 
Tobacco Research.) 
T cells also express an additional transmembrane 
PTPase, LRP. LRP was initially identified by Dr. 
Thomas's group by its similarity to CD45. cDNA se- 
quence analysis predicts a highly glycosylated exte- 
rior domain of 123 amino acids. Dr. Thomas and 
his colleagues developed a monoclonal antibody 
that recognizes the exterior domain of LRP. Use 
of this antibody to immunoprecipitate biosyntheti- 
cally labeled LRP confirms that the protein is post- 
translationally modified by A'- and 0-linked glycosyl- 
ation. Furthermore, staining of cells with anti-LRP 
indicates that LRP is expressed at low levels on many 
cell types, including lymphocytes. Interestingly, 
CD45-deficient cells express LRP, indicating that 
LRP does not compensate for loss of CD45 function. 
The anti-LRP antibody will facilitate the analysis of 
LRP function. 
Dr. Thomas is also Associate Professor of Pathol- 
ogy and Assistant Professor of Molecular Microbi- 
ology at Washington University School of Medi- 
cine, St. Louis. 
Articles 
Fernandez-Luna, J.L., Matthews, R.J., Brownstein, 
B.H., Schreiber, R.D., and Thomas, M.L. 1991. 
Characterization and expression of the human 
leukocyte-common antigen gene (CD45) con- 
tained in yeast artificial chromosomes. Genomics 
Matthews, R.J., Bowne, D.B., Flores, E., and 
Thomas, M.L. 1992. Characterization of hemato- 
poietic intracellular protein tyrosine phospha- 
tases: description of a phosphatase containing an 
SH2 domain and another enriched in proline-, glu- 
tamic acid-, serine-, and threonine-rich se- 
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