By a combination of conventional and recogni- 
tion site affinity chromatography NBP has been pu- 
rified to homogeneity. Greater than 20,000-fold pu- 
rification was achieved, v^^ith an overall yield of 
22%. The purified protein (M^ 63,000) exists as a 
monomer in 0.5 M NaCl. The availability of affinity- 
purified NBP will facilitate functional studies, not 
least by providing peptide sequence information 
that should be useful in identifying the gene for 
NBP 
II. Molecular Mechanisms of Signal Transduction in 
Lymphocyte Activation. 
The programs of lymphocyte proliferation and 
differentiation that underlie the humoral immune 
response are mediated by the binding of three 
classes of ligand— specific antigens, cell-surface 
adhesion molecules, and peptide hormones 
(lymphokines)— to receptors on the surfaces of B 
and T cells. A growing body of evidence implicates 
protein-tyrosine phosphorylation in these pro- 
cesses. By analogy to well -characterized growth fac- 
tor receptors, some of the transmembrane proteins 
implicated in lymphocyte activation may exert their 
effects via specific tyrosine kinases. In the growth 
factor receptor kinases, ligand-binding and kinase 
domains are covalently linked. Similar association 
of transmembrane proteins with cytoplasmic tyro- 
sine kinases of the src type can be achieved by non- 
covalent interactions. 
A. Identification of novel tyrosine kinase genes. 
Seven members of the src family have been de- 
scribed; of these only Ick is preferentially expressed 
in lymphoid cells. The involvement of tyrosine 
phosphorylation in multiple pathways of signal 
transduction in B and T cells suggested the exis- 
tence of additional, lymphoid-specific members of 
the src family. To identify novel cDNA clones encod- 
ing cytoplasmic tyrosine kinases, Dr. Desiderio and 
his colleagues (in collaboration with Dr. John E. 
Niederhuber, The Johns Hopkins Medical School) 
used degenerate oligonucleotide probes to screen 
an activated lymphocyte cDNA library. The probes 
were based on nucleotide sequences conserved 
among known cytoplasmic tyrosine kinases. In a 
screen of 250,000 recombinant bacteriophage, 
seven independent cDNA clones were obtained; by 
cross-hybridization and nucleotide sequence analy- 
sis, these were found to represent six different 
members of the src family. Three of the clones rep- 
resent known genes: src, abl, and Ick. Two others 
represent murine homologues of the genes fyn and 
yes. Significantly, the sixth class represents a novel 
src family member, which Dr. Desiderio and his col- 
leagues have called blk, for B lymphoid kinase. 
B. Identification of a novel protein tyrosine kinase 
expressed specifically in B lymphoid cells. A nearly 
complete blk cDNA has been obtained, and its nu- 
cleotide sequence has been determined. The cDNA 
contains a single long open reading frame of 499 
codons, encoding a 55,000 polypeptide that is 
closely related to but distinct from previously iden- 
tified retroviral and cellular protein-tyrosine ki- 
nases. The amino acid sequence of the protein en- 
coded by blk shares 63% and 61% overall sequence 
identity with the proteins specified by hck and Ick, 
its closest relatives. 
The pattern of expression of blk is unique among 
members of the src family. In immortalized cell 
lines, blk was expressed only in cells of the B lin- 
eage; no expression was seen in cells of T lym- 
phoid, myeloid, erythroid, fibroblastoid, neuronal, 
or hepatocellular origin. In normal murine tissues 
blk transcripts were found only in spleen. Selective 
removal of B cells from the population of normal 
spleen cells resulted in removal of blk transcripts, 
indicating that blk is preferentially expressed in B 
lymphocytes. A survey of cell lines that represent 
various stages in B cell development suggests that 
blk begins to be expressed early in ontogeny, prior 
to expression of immunoglobulin; blk expression 
continues in mature B cells. At present, blk is the 
only known tyrosine kinase gene that is specifically 
expressed in B lymphoid cells. Dr. Desiderio and 
his colleagues propose that the product of blk 
functions in a specialized signal transduction path- 
way in B lymphocytes. Future experiments will ad- 
dress the function of blk and the molecular basis of 
its tissue specificity. 
Dr. Desiderio is also Assistant Professor of Molec- 
ular Biology and Genetics at The Johns Hopkins 
University School of Medicine. 
Continued 
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