dictyNews
Electronic Edition
Volume 38, number 8
March 16, 2012
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Abstracts
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The Molecular Basis of the Dynamic Relocalization of Dictyostelium
Myosin IB
Hanna Brzeska‡, Jake Guag‡, G. Michael Preston‡, Margaret A. Titus§,
and Edward D. Korn‡
From the ‡Laboratory of Cell Biology, National Heart, Lung, and Blood
Institute, National Institutes of Health, 9000 Rockville Pike, Bethesda,
MD 20892 and the
§Department of Genetics, Cell Biology and Development, University of
Minnesota, Minneapolis, MN 55455
J. Biol. Chem., in press
Class-I myosins have a single heavy chain comprising an N-terminal
motor domain with actin-activated ATPase activity and a C-terminal
globular tail with a basic region that binds to acidic phospholipids.
These myosins contribute to the formation of actin-rich protrusions such
as pseudopodia but regulation of the dynamic localization to these
structures is not understood. Previously, we found that Acanthamoeba
myosin IC binds to acidic phospholipids in vitro through a short sequence
of basic and hydrophobic amino acids, BH-site, based on the charge
density of the phospholipids. The tail of Dictyostelium myosin IB (DMIB)
also contains a BH-site. We now report that the BH-site is essential for
DMIB binding to the plasma membrane and describe the molecular basis
of the dynamic relocalization of DMIB in live cells. Endogenous DMIB is
localized uniformly on the plasma membrane of resting cells, at active
protrusions and cell-cell contacts of randomly moving cells, and at the
front of motile polarized cells. The BH-site is required for association of
DMIB with the plasma membrane at all stages where it colocalizes with
PIP2/PIP3. The charge-based specificity of the BH-site allows for in vivo
specificity of DMIB for PIP2/PIP3 similar to the PH domain-based specificity
of other class-I myosins. However, DMIB-head is required for relocalization
of DMIB to the front of migrating cells. Motor activity is not essential but the
actin-binding site in the head is important. Thus, dynamic relocalization of
DMIB is determined principally by the local PIP2/PIP3 concentration in the
plasma membrane and cytoplasmic F-actin.
Submitted by Hanna Brzeska [[log in to unmask]]
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Pleiotropic Roles of a Ribosomal Protein in Dictyostelium discoideum.
Amarnath S, Kawli T, Mohanty S, Srinivasan N, Nanjundiah V.
PLoS One. 2012;7(2):e30644. Epub 2012 Feb 17.
The cell cycle phase at starvation influences post-starvation differentiation
and morphogenesis in Dictyostelium discoideum. We found that when
expressed in Saccharomyces cerevisiae, a D. discoideum cDNA that
encodes the ribosomal protein S4 (DdS4) rescues mutations in the cell
cycle genes cdc24, cdc42 and bem1. The products of these genes affect
morphogenesis in yeast via a coordinated moulding of the cytoskeleton
during bud site selection. D. discoideum cells that over- or under-expressed
DdS4 did not show detectable changes in protein synthesis but displayed
similar developmental aberrations whose intensity was graded with the
extent of over- or under-expression. This suggested that DdS4 might
influence morphogenesis via a stoichiometric effect - specifically, by taking
part in a multimeric complex similar to the one involving Cdc24p, Cdc42p
and Bem1p in yeast. In support of the hypothesis, the S. cerevisiae proteins
Cdc24p, Cdc42p and Bem1p as well as their D. discoideum cognates could
be co-precipitated with antibodies to DdS4. Computational analysis and
mutational studies explained these findings: a C-terminal domain of DdS4
is the functional equivalent of an SH3 domain in the yeast scaffold protein
Bem1p that is central to constructing the bud site selection complex. Thus
in addition to being part of the ribosome, DdS4 has a second function, also
as part of a multi-protein complex. We speculate that the existence of the
second role can act as a safeguard against perturbations to ribosome
function caused by spontaneous variations in DdS4 levels.
Submitted by Smita Amarnath [[log in to unmask]]
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[End dictyNews, volume 38, number 8]
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