CSM News
Electronic Edition
Volume 1, number 16
Sept. 25, 1993

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

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ABSTRACTS
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Accumulation of unsulphated precursors in Dictyostelium discoideum during 
selenate inhibition of growth.

S. Schmidt and J.F. Wheldrake, School of Biological Sciences, Flinders 
University, Bedford Park, S.Australia 5042.

Molecular and Cellular Biochemistry, in press

ABSTRACT

       Incubation of Dictyostelium discoideum cells with selenate is known 
to inhibit vegetative growth. In this paper we show that in the presence of 
selenate macromolecules accumulate which can be converted to sulphated 
products once the selenate is removed. The presence of cycloheximide, an 
inhibitor of protein synthesis, during the subsequent incubation does not 
prevent this conversion but tunicamycin, an inhibitor of glycosylation, 
does. It is concluded that, in the presence of selenate, precursors 
accumulate as unglycosylated proteins, suggesting that feedback inhibition 
of glycosylation may be operated.

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Cell density-sensing in Dictyostelium by means of the accumulation
rate, diffusion coefficient and activity threshold of a protein
secreted by starved cells

Ita S. Yuen and Richard H. Gomer

Howard Hughes Medical Institute and Department of Biochemistry and
Cell Biology, Rice University, Houston, TX 77251-1892

Journal of Theoretical Biology, in press

Abstract

     The simple eukaryote Dictyostelium discoideum grows as an amoeba
on leaf and soil surfaces.  When starved, the amoebae aggregate and
differentiate.  The amoebae can also be induced to differentiate as
isolated cells submerged in buffer, if the buffer contains a
sufficiently high concentration of a protein (CMF) secreted by starved
cells.  CMF is also necessary for aggregation and differentiation on
surfaces.  This indicated that CMF has either an autocrine function or
is part of a density-sensing system.  To distinguish between these two
possible functions, we first examined the rate at which CMF is
accumulated and the activity threshold of cells for CMF, since both
parameters will affect whether a cell can provide enough CMF to
self-stimulate.  We find that CMF potentiates its own accumulation,
and that otherwise the accumulation rate and activity threshold are
affected very little by a variety of physiological conditions.  We
then use diffusion calculations to show that even after many hours of
continuous secretion, the CMF concentration adjacent to an isolated
starved cell on a leaf or soil surface will be too low to allow
differentiation, whereas an extracellular concentration of CMF
sufficiently high to allow differentiation will occur when starved
cells are at high densities.  We find a close match between the
predicted and experimentally observed density necessary for
differentiation.  The theoretical and observed behavior of cells at
different cell densities suggests that due to its accumulation rate,
diffusion coefficient, and activation threshold, CMF can function as
part of a cell density-sensing system which allows Dictyostelium cells
in the wild to coordinate their development.

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