Molecular and Cellular Biology, August 2000, p. 5917-5929, Vol. 20, No. 16
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Homodimerizes on the Cell Surface

Molecular Biology and Virology Laboratory, The Salk Institute for Biological Studies, La Jolla, California 92037,1 and Hubrecht Laboratory, Netherlands Institute for Developmental Biology, 3584 CT Utrecht, The Netherlands2
Received 28 December 1999/Returned for modification 13 March 2000/Accepted 12 May 2000
We reported previously that the N-terminal D1 catalytic domain of
receptor protein-tyrosine phosphatase
(RPTP
) forms a symmetrical, inhibited dimer in a crystal structure, in which a
helix-turn-helix wedge element from one monomer is inserted into the
catalytic cleft of the other monomer. Previous functional studies also
suggested that dimerization inhibits the biological activity of a CD45
chimeric RPTP and the catalytic activity of an isolated RPTP
D1
catalytic domain. Most recently, we have also shown that enforced
dimerization inhibits the biological activity of full-length
RPTP
in a wedge-dependent manner. The physiological significance of
such inhibition is unknown, due to a lack of understanding of how
RPTP
dimerization is regulated in vivo. In this study, we show that
transiently expressed cell surface RPTP
exists predominantly as
homodimers, suggesting that dimerization-mediated inhibition of
RPTP
biological activity is likely to be physiologically relevant.
Consistent with our published and unpublished crystallographic data, we
show that mutations in the wedge region of D1 catalytic domain and
deletion of the entire D2 catalytic domain independently reduced but
did not abolish RPTP
homodimerization, suggesting that both domains are critically involved but that neither is essential for
homodimerization. Finally, we also provide evidence that both the
RPTP
extracellular domain and the transmembrane domain were
independently able to homodimerize. These results lead us to propose a
zipper model in which inactive RPTP
dimers are stabilized by
multiple, relatively weak dimerization interfaces. Dimerization in
this manner would provide a potential mechanism for negative regulation
of RPTP
. Such RPTP
dimers could be activated by extracellular
ligands or intracellular binding proteins that induce monomerization or by intracellular signaling events that induce an open conformation of
the dimer.
Present address: Molecular Endocrinology and Metabolic Disorders,
Merck & Co., Inc., Rahway, NJ 07065.
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