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REVERSIBLE PROTEIN PHOSPHORYLATION 273
CK1. These intriguing results suggest that the
parasite CK1s exhibit high affinities towards
purvalanols. L. donovani CK1 in promastigotes
is an ecto-protein kinase that is shed into the
medium.
Protein phosphatases
Protein phosphatases are required to finely
regulate the duration and magnitude of
protein phosphorylation, and consequently to
modulate the activation/deactivation of the
signaling processes. Protein phosphatases are
divided into three types: serine/threonine
phosphatases specific for phosphorylated
serine and threonine residues, tyrosine phosphatases
specific for phosphotyrosine residues,
and dual-specificity phosphatases that dephosphorylate
phospho-serine/threonine and phosphotyrosine
residues. All these phosphatases
have unique structural characteristics and can
be divided into different subfamilies. Phosphoserine/threonine-specific
phosphatases can
be subdivided into PPP and PPM families.
Protein phosphatases PP1, PP2A, PP2B (calcineurin),
PP4, PP5 and PP6 belong to the PPP
subfamily. The catalytic domains of these phosphatases
are quite diverse and often comprise
the majority of the catalytic subunit. The PPM
subfamily includes Mg 2 -dependent PP2C
phosphatases. Substrate specificity of these
phosphatases is controlled by a diverse group of
regulatory subunits. All protein tyrosine phosphatases
(PTPs) contain a conserved cysteine
in the catalytic domain that is important for
activity. Other regions in the PTPs are important
for subcellular localization. PTPs can be integral
membrane proteins, cytosolic type, or nontransmembrane
type. The dual-specific phosphatases
include vaccinia virus VH1 protein
and its mammalian homolog VHR, and MAP
kinase phosphatases (MKPs), among others.
Protein phosphatases from apicomplexans
Compared to efforts devoted to protein kinases,
studies on protein phosphatases in protozoan
parasites are not as extensive. Nonetheless, a
wide variety of phosphatases appear to be used
to regulate the growth and differentiation of
medically important parasites. In P. falciparum,
inhibitors of PP1 and PP2A (okadaic acid and
calyculin, respectively) block intraerythrocytic
growth. Since erythrocytes also have significant
phosphatase activity, it is not clear whether this
inhibition of parasite growth is due, to some
extent, to inhibition of red cell metabolism.
PfPP2A-like activity in Plasmodium is sensitive
to low nanomolar concentrations of okadaic
acid. In contrast, the PP2B-like activity is inhibited
by the cyclosporin–cyclophilin complex,
but is insensitive to okadaic acid. The peptide
sequence derived from amino terminal microsequencing
of a putative 35 kDa PfPP2A shows
homology with PP2A subfamily sequences in the
database. Using the amino terminal peptide
sequence, a 309-residue PP2A ORF has been
identified from the malaria genome database.
The PfPP2A sequence contains all the signature
motifs of serine/threonine phosphatases,
including the GDXHGQ, GDXVDRG and RGNHE
sequences important for catalysis, substrate
binding, or metal-ion binding. PfPP2A also
contains the SAPNYCYRCG motif essential for
high-affinity binding of okadaic acid.
Another P. falciparum PP2A-like phosphatase,
termed PP-, has a catalytic domain
simlar to PfPP2A. However, PfPP contains an
amino terminal extension and is detected only
in the sexual stages of the parasite. Recently, a
novel protein phosphatase, PfPPJ, of the PPP
superfamily has been characterized. The predicted
primary amino acid sequence of PfPPJ
contains signature sequences of the PPP
superfamily, but has a unique carboxy terminal
sequence and unique sensitivity to inhibitors.
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA