Dicty News
Electronic Edition
Volume 10, number 2
January 10, 1998

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

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

===========
 Abstracts
===========

Dictyostelium gamma-tubulin: molecular characterization and 
ultrastructural localization

Ursula Euteneuer, Ralph Graef, Eckhard Kube-Granderath, and Manfred 
Schliwa

J. Cell Sci., in press

Summary

   The centrosome of Dictyostelium discoideum is a nucleus-associated 
body consisting of an electron-dense, three-layered core surrounded by 
an amorphous matrix, the corona. To elucidate the molecular and 
supramolecular architecture of this unique microtubule-organizing 
center, we have isolated and sequenced the gene encoding gamma-tubulin 
and have studied its localization in the Dictyostelium centrosome using 
immunofluorescence and postembedding immunoelectron microscopy. D. 
discoideum possesses a single copy of a gamma-tubulin gene that is 
related to, but more divergent from, other gamma-tubulins. The low-
abundance gene product is localized to the centrosome in an intriguing 
pattern: it is highly concentrated in the corona in regularly spaced 
clusters whose distribution correlates with the patterning of dense 
nodules that are a prominent feature of the corona.  These observations 
lend support to the notion that the corona is the functional homologue 
of the pericentriolar matrix of higher eukaryotic centrosomes, and 
that nodules are the functional equivalent of gamma-tubulin ring 
complexes that serve as nucleation sites for microtubules in animal 
centrosomes.

-------------------------------------------------------------------------

Phosphorylation of the Dictyostelium Myosin II Heavy Chain is Necessary 
for Maintaining Cellular Polarity and Suppressing Turning During
Chemotaxis

Janice Stites1, Deborah Wessels1, Amanda Uhl1, Thomas Egelhoff*, Damon 
Shutt1 and David R. Soll1

1Department of Biological Sciences University of Iowa, Iowa City, 
IA 52242  *Department of Physiology and Biophysics, Case Western 
Reserve School of Medicine Cleveland, OH 44106

Cell Motility and the Cytoskeleton, in press.

Conversion of the three mapped threonine phosphorylation sites in the 
myosin II heavy chain tail to alanines results in a mutant (3XALA) in 
Dictyostelium discoideum which displays constitutive 
myosin overassembly in the cytoskeleton and increased cortical tension.  
To assess the importance of myosin phosphorylation in cellular 
translocation and chemotaxis, 3XALA mutant cells have been analyzed 
by 2D and 3D computer-assisted methods in buffer, in a spatial gradient 
of cAMP and after the rapid addition of cAMP.  3XALA cells crawling in 
buffer exhibit distinct abnormalities in cellular shape, the maintenance 
of polarity and the complexity of the pseudopod perimeter.  3XALA cells 
crawling in buffer also exhibit a decrease in directionality.  In a 
spatial gradient of cAMP, the behavioral defects are accentuated.  In a 
spatial gradient, 3XALA cells exhibit a repeating one to two minute 
behavior cycle in which the shape of each cell changes abnormally from 
elongate to extremely wide with lateral, opposing pseudopods.  At the 
end of each cycle, 3XALA cells turn 90° into the left or right lateral 
pseudopod, resulting in a dramatic depression in chemotactic efficiency, 
even though 3XALA cells are chemotactically responsive to cAMP.  These 
results demonstrate that the phosphorylation of myosin II heavy chain 
plays a critical role in the maintenance of cell shape and in persistent 
translocation in a spatial gradient of chemoattractant.

-------------------------------------------------------------------------
[End Dicty News, volume 10, number 2]