dictyNews
Electronic Edition
Volume 38, number 8
March 16, 2012

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

The Molecular Basis of the Dynamic Relocalization of Dictyostelium 
Myosin IB

Hanna Brzeskaā, Jake Guagā, G. Michael Prestonā, Margaret A. TitusĪ, 
and Edward D. Kornā

From the āLaboratory of Cell Biology, National Heart, Lung, and Blood 
Institute, National Institutes of Health, 9000 Rockville Pike, Bethesda, 
MD 20892 and the 
ĪDepartment 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 [brzeskah@nhlbi.nih.gov]
--------------------------------------------------------------------------------------


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 [smita.smitar@gmail.com]
==============================================================
[End dictyNews, volume 38, number 8]