The
Unit on Molecular Signal Transduction investigates signal transduction pathways
that mediate the actions of hormones and growth factors in mammalian cells,
with special emphasis on the role of phosphoinositide-derived messengers.
Current studies are aimed at understanding the function and regulation of
several phosphatidylinositol (PI) 4-kinases in the control of the synthesis
of hormone-sensitive phosphoinositide pools, characterizing the structural
features that determine the catalytic specificity and inhibitor sensitivity
of PI 3- and PI 4-kinases, defining the molecular basis of protein-phosphoinositide
interactions via the pleckstrin homology domain and other domains of selected
regulatory proteins, developing tools to analyze inositol lipid dynamics
in live cells, and determining the importance of lipid-protein interactions
in the activation of cellular responses by G protein-coupled receptors and
receptor tyrosine kinases.
Neuronal Calcium Sensor-1 Regulates Phosphatidylinositol 4-Kinase
Beta in Mammalian Cells
Zhao,a A Balla, Tujmetova, Várnai,
T Balla
Inositol lipid kinases are increasingly recognized as regulators of membrane
remodeling events, including Golgi-to-plasma membrane transport, exocytosis,
or endocytosis. The enzymes catalyze the formation of specific inositol
phospholipids that contribute to the membrane recruitment and stabilization
of molecular complexes via interaction of inositides with protein motifs
present in several regulatory proteins. PI 4-kinases (PI4Ks) are the enzymes
that catalyze the formation of PI(4)P, the main precursor of several other
polyphosphoinositides responsible for important regulatory functions.
Investigators in this unit have recently purified and cloned two mammalian
PI4Ks, a larger (~200 kDa) alpha and a smaller (~100 kDa) beta, form from
bovine adrenal and brain. The enzymes are mammalian homologs of the yeast
STT4 and PIK1 gene products, respectively, and are highly conserved in
all eukaryotes, including plants. Recently, it has been reported that
the yeast homolog of the Ca2+-dependent regulatory protein,
NCS-1, is able to stimulate PI 4-kinase activity of yeast homogenates,
apparently through interaction with the Pik1 protein. NCS-1 was first
identified in Drososphila (where it was named frequenin) as an
important determinant of synaptic plasticity and a regulator of synaptic
development. Homologs of NCS-1 have been found in Xenopus as well
as in avian and mammalian tissues and, together with recoverin/neurocalcin,
form a group of small Ca2+-binding proteins distinct from calmodulin.
We thus investigated whether mammalian NCS-1 is able to interact with
and regulate PI4Kbeta in mammalian cells. Recombinant PI4Kbeta, but not
its GST-fused form, showed enhanced PI kinase activity when incubated
with recombinant NCS-1, but only if the latter was myristoylated. Similarly,
in vitrotranslated NCS-1, but not its myristoylation-defective mutant,
was found to be associated with recombinant- or in vitro-translated PI4Kbeta
in PI4Kbeta-immunoprecipitates. When expressed in COS-7 cells, PI4Kbeta
and NCS-1 formed a complex that could be immunoprecipitated with antibodies
against either protein, and PI 4-kinase activity was present in anti-NCS-1
immunoprecipitates. Confocal analysis of the distribution of expressed
NCS-1-YFP showed that NCS-1 is colocalized with endogenous PI4Kbeta primarily
in the Golgi but is also present in the plasma membrane and the walls
of numerous large perinuclear vesicles that are not observed in untransfected
COS-7 cells. Coexpression of a catalytically inactive PI4Kbeta inhibited
the development of the vesicular phenotype, suggesting that the vesicles
are formed as the consequence of NCS-1 activating PI4Kbeta. Transfection
of PI4Kbeta and NCS-1 had no effect on basal PIP synthesis in permeabilized
COS-7 cells, whereas it increased the wortmannin-sensitive 32P-phosphate
incorporation into phosphatidylinositol 4-phosphate during Ca2+-induced
phospholipase C activation. Together, the results indicate that NCS-1
is able to interact with PI4Kbeta in mammalian cells and may play a role
in the regulation of the enzyme in specific cellular compartments affecting
vesicular trafficking.
Analysis of Inositol Phospholipid Changes in Relation to Activation of
the Small GTP Binding Protein Arf6
T. Balla in collaboration with J. Donaldson and F. Brown (NIAID)
Activation of small GTP binding proteins is often correlated with changes
in the level of inositol phospholipids. A number of guanine nucleotide
exchange factors (GEFs) and GTPase activating proteins (GAPs) are activated
by PI(3,4,5)P3, the product of PI 3-kinases,
and some by PI(4,5)P2. In collaboration with
investigators in NHLBI (led by Dr. Julie Donaldson), we investigated the
role of PI(4,5)P2 in plasma membrane (PM)
dynamics regulated by ADP-ribosylation factor Arf6. Arf6 activates phosphatidylinositol
4-phosphate 5-kinase (PIP 5-kinase), an enzyme that generates PI(4,5)P2,
and we used the pleckstrin homology domain of PLCdelta fused to the green
fluorescent protein (PLCd1PH-GFP) to visualize the lipid during Arf6 activation.
Activation of Arf6 by expression of its exchange factor EFA6 stimulated
formation of membrane protrusions and the uptake of PM into macropinosomes
enriched in PI(4,5)P2 as well as the recycling
of the membrane back to the PM. By contrast, expression of Arf6 Q67L,
a GTP hydrolysis-resistant mutant, induced the formation of PIP2-positive
actin-coated vacuoles that were unable to recycle membrane back to the
PM. Overexpression of human PIP 5-kinase alpha mimicked the effects seen
with Arf6 Q67L. The results demonstrate that PIP 5-kinase activity and
PIP2 turnover controlled by activation and inactivation of Arf6 is critical
for trafficking through the Arf6 PM-endosomal recycling pathway.
Analysis of Ras Activation by Novel Fluorescent Tools
Bondeva,a T. Balla, Várnai,
Barshishat, A. Balla
We also explored the possibility of developing additional research tools
to analyze the role of inositol lipids in the regulation of another important
small GTP binding protein, Ras. It has been shown that H-Ras is present
in special membrane subdomains, termed RAFTs, that are also enriched in
inositol phospholipids. The active, GTP-bound form of Ras has been shown
to activate PI 3-kinases while PI(3,4,5)P3
has been shown to recruit both GEFs and GAPs that regulate the active
state of Ras. To study the dynamics of Ras activation in live cells and
explore its connection with RAFTs and the various inositol lipids, we
investigated whether the minimum molecular determinants of Ras recognition
by the Raf-1 serine/threonine kinase, the best-known downstream target
of Ras, could be used to visualize Ras activation in live cells by following
the distribution of such domain fused to the green fluorescent protein
(GFP). When the Ras binding domain (RBD) of Raf-1 was fused to GFP [Raf-1(51-131)GFP],
we observed very little localization of fluorescence in the plasma membrane
of Ras-transformed NIH 3T3 cells. However, addition of the cystein-rich
region (CRD) to the construct [Raf-1(51-220)GFP] showed clear localization
to membrane ruffles of Ras-transformed NIH 3T3 cells. In normal NIH 3T3
cells, [Raf-1(51-220)GFP] showed minimal membrane localization, which
was nonetheless enhanced after stimulation with PDGF or PMA. Mutations
within either the RBD (R89L) or CRD (C168S) disrupted the membrane localization
of [Raf-1(51-220)GFP], suggesting that both domains contribute to the
recruitment of the fusion protein to Ras at the plasma membrane. The abilities
of the various constructs to localize to the plasma membrane closely correlated
with their inhibitory effects on MEK1- or MAP-kinase activation. Membrane
localization of full-length Raf-1-GFP was less prominent than that of
[Raf-1(51-220)GFP] despite the strong binding of the former to RasV12
and potent activation of MAP-kinase. The findings indicate that both RBD
and CRD are needed to recruit Raf-1 to active Ras at the plasma membrane
and that these domains are not fully exposed in the Raf-1 molecule. Visualization
of activated Ras in live cells will help us more fully understand the
dynamics of Ras activation under various physiological and pathological
conditions and, combined with our other GFP-fused domains that recognize
phosphoinositides, will permit analysis of the role of inositides in Ras
activation.
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PUBLICATIONS
- Balla
T. Pharmacology of phosphoinositides, regulators of multiple cellular
functions. Curr Pharm Design 2001;7:475-507.
- Brown
FD, Rozelle AL, Yin HL, Balla T, Donaldson JG. Phosphatidylinositol
4,5-bisphosphate and Arf6-regulated membrane traffic. J Cell Biol 2001;154:1007-1018.
- Marshall
JG, Booth JW, Stambolic V, Mak T, Balla T, Schreiber AD, Meyer T, Grinstein
S. Restricted accumulation of phosphatidylinositol 3-kinase products
in a plasmalemmal subdomain during Fc gamma receptor-mediated phagocytosis.
J Cell Biol 2001;153:1369-1380.
- Van
der Wal J, Habets R, Varnai P, Balla T, Jalink K. Monitoring phospholpiase
C activation kinetics by FRET. J Biol Chem 2001;276:15337-15344.
- Zhao
X-H, Várnai P, Tuymetova G, Balla A, Tóth Z, Oker-Blom
C, Roder J, Jeromin A, Balla T. Interaction of neuronal calcium
sensor-1(NCS-1) with phosphatidylinositol 4-kinase beta stimulates lipid
kinase activity and affects membrane trafficking in COS-7 cells. J Biol
Chem 2001;276:40183-40189.
- Zhou
X, Jiang G, Zhao A, Bondeva T, Hirszel P, Balla T. Inhibition of
Na,K-ATPase activates PI3 kinase and inhibits apoptosis in LLC-PK1 cells.
Biochem Biophys Res Commun 2001;285:46-51.
aX-H. Zhou, and T. Bondeva. were visiting
fellows who left in September 2000.
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