Copy Link
Add to Bookmark
Report
dictyNews Volume 17 Number 14
Dicty News
Electronic Edition
Volume 17, number 14
December 15, 2001
Please submit abstracts of your papers as soon as they have been
accepted for publication by sending them to dicty@northwestern.edu.
Back issues of Dicty-News, the Dicty Reference database and other useful
information is available at DictyBase--http://dictybase.org.
===========================
New Report on DictyBase
===========================
John Bonner has contributed a short report on the number of cells in
Dictyostelium slugs. You can download this report from
http://dictybase.org.
=============
Abstracts
=============
The Formation of Actin Rods Composed of Actin Tubules in Dictyostelium
discoideum Spores
Masazumi Sameshima,*1 Yoshiro Kishi,* Masako Osumi, Reiko Minamikawa-Tachino,
Dana Mahadeo,2 and David A. Cotter2
*Electron Microscopy Center, and Computer Center, The Tokyo Metropolitan
Institute of Medical Science, Tokyo Metropolitan Organization for Medical
Research, 3-18-22 Honkomagome, Bunkyo-ku, Tokyo 113-8613, Japan; Department
of Chemical and Biological Sciences, Faculty of Science, Japan Women's
University, Bunkyo-ku, Tokyo 112-8681, Japan; and 2Department of Biological
Sciences, University of Windsor, Windsor, Ontario N9B 3P4, Canada
Accepted by J Struct Biol
Abstract
A new type of actin rods formed in both of the nucleus and the cytoplasm
as well as tyrosine phosphorylation of actin are implicated in the
maintenance of dormancy and viability of Dictyostelium discoideum spores.
Recently we have found that S-adenosyl-L-homocysteine hydrolase that is a
key enzyme for methylation is sequestered into actin rods. Here the
ultrastructure of the rods and their relationship to the phosphorylation
were examined. The rods first appeared in premature spores at the mid
culmination stage as bundles composed of actin tubules hexagonally
cross-linked. The 13 nm diameter bundles were composed of three actin
filaments. Formation of the actin rods begins during late culmination
stage and proceeds until two days after completion of fruiting bodies.
The physical events occur in the following order; association of several
modules of bundles, closely packing and decrease in diameter of actin
tubules, elongation of rods across the nucleus or the cytoplasm. Actin
phosphorylation levels increased at the late culmination stage and reached
a maximum level 12 hours later. Immediately following activation of spore
germination, actin was rapidly dephosphorylated, followed shortly thereafter
with the disappearance of rods. Shortened actin tubules once again became
arranged in a hexagonal pattern. This hexagonal arrangements of actin
tubules are possibly involved in rod formation and disappearance, and do
not depend upon actin phosphorylation. In contrast, rod-maturation processes
may correlate with actin phosphorylation.
-----------------------------------------------------------------------------
REGULATION OF ADENYLYL CYCLASES BY A REGION OUTSIDE THE MINIMALLY FUNCTIONAL
CYTOPLASMIC DOMAINS
Carole A. Parent, Jane Borleis, and Peter N. Devreotes
Department of Biological Chemistry, The Johns Hopkins School of Medicine,
Baltimore, MD 21205
Journal of Biological Chemistry, in press.
The highly conserved topological structure of G protein activated adenylyl
cyclases seems unnecessary since the soluble cytoplasmic domains retain
regulatory and catalytic properties. Yet, we previously isolated a
constitutively active mutant of the D. discoideum adenylyl cyclase harboring
a single point mutation in the region linking the cytoplasmic and membrane
domains (L394). We show here that multiple amino acid substitutions at L394
also display constitutive activity. The constitutive activity of these
mutants is not dependent on G proteins or cytosolic regulators, although
some of the mutants can be activated to higher levels than wild type.
Combining a constitutive mutation such as L394T with K482N, a point mutation
that renders the enzyme insensitive to regulators, restores an enzyme with
wild type properties of low basal activity and the capacity to be activated
by G proteins. Thus regions located outside the cytoplasmic loops of
adenylyl cyclases are not only important in the acquisition of an activated
conformation, they also have impact on other regions within the catalytic
core of the enzyme.
-----------------------------------------------------------------------------
Phospholipase Cd regulates germination of Dictyostelium spores
Peter Van Dijken and Peter J.M. Van Haastert
Department of Biochemistry, University of Groningen, Nijenborgh 4, 9747 AG
Groningen, The Netherlands
BMC Cell Biology, in press.
Abstract
Many eukaryotes, including plants and fungi make spores that resist severe
environmental stress. The micro-organism Dictyostelium contains a single
phospholipase C gene (PLC); deletion of the gene has no effect on growth,
cell movement and differentiation. In this report we show that PLC is
essential to sense the environment of food-activated spores. Plc-null spores
germinate at alkaline pH, reduced temperature or increased osmolarity,
conditions at which the emerging amoebae can not grow. In contrast,
food-activated wild-type spores return to dormancy till conditions in the
environment allow growth. The analysis of inositol 1,4,5-trisphosphate (IP3)
levels and the effect of added IP3 uncover an unexpected mechanism how PLC
regulates spore germination: i) deletion of PLC induces the enhanced activity
of an IP5 phosphatase leading to high IP3 levels in plc-null cells; ii) in
wild-type spores unfavourable conditions inhibit PLC leading to a reduction
of IP3 levels; addition of exogenous IP3 to wild-type spores induces
germination at unfavourable conditions; iii) in plc-null spores IP3 levels
remain high, also at unfavourable environmental conditions. The results
imply that environmental conditions regulate PLC activity and that IP3
induces spore germination; the uncontrolled germination of plc-null spores
is not due to a lack of PLC activity but to the constitutive activation of
an alternative IP3- forming pathway.
-----------------------------------------------------------------------------
[End Dicty News, volume 17, number 14]
