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We have used in vitro mutagenesis to examine in detail the roles of two modular
protein domains, SH2 and SH3, in the regulation of the Abl tyrosine kinase. As
previously shown, the SH3 domain suppresses an intrinsic transforming activity
of the normally nontransforming c-Abl product in vivo. We show here that this
inhibitory activity is extremely position sensitive, because mutants in which
the position of the SH3 domain within the protein is subtly altered are fully
transforming. In contrast to the case in vivo, the SH3 domain has no effect on
the in vitro kinase activity of the purified protein. These results are
consistent with a model in which the SH3 domain binds a cellular inhibitory
factor, which in turn must physically interact with other parts of the kinase.
Unlike the SH3 domain, the SH2 domain is required for transforming activity of
activated Abl alleles. We demonstrate that SH2 domains from other proteins
(Ras-GTPase-activating protein, Src, p85 phosphatidylinositol 3-kinase subunit,
and Crk) can complement the absence of the Abl SH2 domain and that mutants with
heterologous SH2 domains induce altered patterns of tyrosine-phosphorylated
proteins in vivo. The positive function of the SH2 domain is relatively position
independent, and the effect of multiple SH2 domains appears to be additive.
These results suggest a novel mechanism for regulation of tyrosine kinases in
which the SH2 domain binds to, and thereby enhances the phosphorylation of, a
subset of proteins phosphorylated by the catalytic domain. Our data also suggest
that the roles of the SH2 and SH3 domains in the regulation of Abl are different
in several respects from the roles proposed for these domains in the closely
related Src family of tyrosine kinases.
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