CSM News
Electronic Edition
Volume 8, number 13
May 17, 1997

Please submit abstracts of your papers as soon as they have been
accepted for publication by sending them to CSM-News@nwu.edu.

Back issues of CSM-News, the CSM Reference database and other useful
information is available at the Dictyostelium Web Page 
"http://dicty.cmb.nwu.edu/dicty/dicty.html"

===========================
 Dicty Researcher Database
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Also, please if you have not completed your entry in the Dicty 
Researcher Database on the Dicty web page or by following this 
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===========
 Abstracts
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The Dictyostelium MAP kinase kinase DdMEK1 regulates chemotaxis and is
essential for chemoattractant-mediated activation of guanylyl cyclase

Hui Ma, Marianne Gamper, Carole Parent, and Richard A. Firtel

EMBO J., in press.

ABSTRACT

        We have identified a MAP kinase kinase (DdMEK1) that is required
for proper aggregation in Dictyostelium.  Null mutations produce extremely
small aggregate sizes, resulting in the formation of slugs and terminal
fruiting bodies that are significantly smaller than those of wild-type
cells.  Time-lapse video microscopy and in vitro assays indicate that the
cells are able to produce cAMP waves that move through the aggregation
domains.  However, these cells are unable to properly chemotax during
aggregation in response to the chemoattractant cAMP or activate guanylyl
cyclase, a known regulator of chemotaxis in Dictyostelium.  The activation
of guanylyl cyclase in response to osmotic stress is, however, normal.
Expression of putative constitutively active forms of DdMEK1 in a ddmek1
null background is capable of at least partially complementing the small
aggregate size defect and the ability to activate guanylyl cyclase;
however, this does not result in constitutive activation of guanylyl
cyclase, suggesting that DdMEK1 activity is necessary, but not sufficient,
for cAMP activation of guanylyl cyclase.  Analysis of a
temperature-sensitive DdMEK1 mutant suggests that DdMEK1 activity is
required throughout aggregation at the time of guanylyl cyclase activation
but is not essential for proper morphogenesis during the later
multicellular stages.  The activation of the MAP kinase ERK2, which is
essential for chemoattractant activation of adenylyl cyclase, is not
affected in ddmek1 null strains, indicating that DdMEK1 does not regulate
ERK2 and suggesting that at least two independent MAP kinase cascades
control aggregation in Dictyostelium.
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Chemoattractants induce tyrosine phosphorylation of ERK2 by diverse
signalling pathways.

Chiya Kosaka and Catherine Pears. 

Biochem. J. In press

   Two homologues of mitogen activated protein kinases have been identified
in Dictyostelium (ERK1 and ERK2). We demonstrate transient tyrosine
phosphorylation of ERK2 in response to the chemoattractants cAMP and folic
acid that correlates with activity. To investigate the signalling
pathways, we studied the response in strains with altered cAMP-dependent
protein kinase (PKA) status. The degree of cAMP-induced ERK2 tyrosine
phosphorylation was increased in cells overexpressing PKA activity but no
such increase was observed in response to folic acid. Our observations
suggest that cAMP-induced tyrosine phosphorylation is positively modulated
by a PKA-regulated step which is not involved in the response to folic
acid, suggesting the presence of diverse signalling pathways leading to
ERK2 phosphorylation. 

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[End CSM News, volume 8, number 12]