trans

CALCIUM 251
Similar results are obtained with insect forms
of T. brucei, intracellular forms of T. cruzi and
promastigote forms of L. donovani. The Ca 2
influx exhibits some of the hallmarks of channel
activity, including selectivity for Ca 2 over
other divalent cations, and inhibition by heavy
metals. The process does not require emptying
of stored Ca 2 from the ER or the acidocalcisome.
Conversely, release of Ca 2 from the
acidocalcisome with nigericin or monensin in
intact cells does not result in Ca 2 influx across
the plasma membrane. Therefore, Ca 2 influx
in T. brucei does not appear to involve the
capacitative process. Instead, phospholipase
activity is required for Ca 2 influx.
Phospholipase A 2 inhibitors block the effect
of melittin, while free arachidonic acid mimics
the effect of melittin. In addition, P. falciparum
and T. brucei are each capable of producing
further lipid signals in the form of prostaglandins
F 2 and D 2 , as well as arachidonic
acid. The prospect of lipid-based signals generated
by these parasites opens up new areas
for future research.
Ca 2 -binding proteins
Low affinity proteins
Cells respond to Ca 2 signals in different ways.
The complement of Ca 2 -binding proteins
determines the repertoire of cellular responses.
In general, Ca 2 -binding involves oxygen atoms
that are provided by acidic side chains in amino
acids, the carbonyl of the peptide bond, or
water within the Ca 2 -binding site. Low affinity
Ca 2 -binding proteins typically use repeats
of acidic residues to bind Ca 2 . Along with
the low affinity Ca 2 -binding proteins of the
ER, both T. brucei and P. falciparum contain a
homolog of the translationally controlled tumor
protein. Ca 2 -binding has been confirmed by
45 Ca-overlay procedures. The P. falciparum
protein has been localized to the cytoplasm
and to food vacuoles. Although the function of
the translationally controlled tumor protein is
not known, it has been identified as the primary
binding target for the anti-malaria drug
artemisinin, and therefore may be essential
for cell survival.
High affinity Ca 2 -binding proteins
Annexins
In contrast with low affinity Ca 2 -binding proteins,
the high affinity Ca 2 -binding proteins
utilize well defined motifs to coordinate with
Ca 2 . Annexins are an extensive family of
Ca 2 - and phospholipid-binding proteins that
share conserved repeats and an annexin consensus
sequence. In mammalian cells, annexins
play essential roles in the mediation of
cytoskeleton organization, secretion, and regulation
of ion channels. Considering the broad
range of annexin activities, it is surprising that
only one member of the annexin family has
been described in any protozoan parasite. The
Giardia protein alpha 1-giardin has been
classified as annexin XIX. Its role is not well
understood, but it presumably mediates microtubule
organization in the ventral disk. Limited
numbers of annexin family members appear
to be present in the genomes of Plasmodium,
T. gondii and T. cruzi, but not T. brucei. Procedures
that biochemically detect annexins in
mammalian cells are not successful in detecting
annexins in T. brucei. Why the protozoan
parasites appear to have limited numbers of
annexin family members is not known.
Protein kinase C
Protein kinase C (PKC) is another extensive
family of phospholipid- and Ca 2 -binding
proteins. Various isoforms are classified as
‘classical’ (bind Ca 2 , phosphatidylserine and
diacylglycerol), ‘novel’ (lack the Ca 2 -binding
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA