trans

176 PROTEIN METABOLISM
and also, to a lesser extent, at the outer cell
surface.
T. vaginalis secretes a complex mixture of
cysteine proteinases, with apparent molecular
masses ranging from 20 to 96 kDa. However,
cloning and sequencing of several of these
cysteine proteinase genes has not predicted
mature enzymes larger than 24 kDa. These
sequences are more similar to mammalian
cathepsin L than to cruzipain or the Type I
and II cysteine proteinases from L. mexicana.
T. vaginalis enzymes have both cathepsin
L-like and cathepsin B-like specificity. The
cysteine proteinases in the cattle parasite
Tritrichomonas foetus appear to be similar to
those in T. vaginalis. T. foetus produces and
secretes several cysteine proteinases into the
medium, and seven different genes have been
cloned and sequenced.
G. lamblia trophozoites contain at least
18 proteolytic activities with apparent molecular
mass values from 30 to 211 kDa, with responses
to inhibitors suggesting that many are cysteine
proteinases. These activities appear to be
lysosomal. Recently, McKerrow and co-workers
have cloned and sequenced a cysteine proteinase,
with homology to cathepsin B, that is
required for excystation of the parasite.
Serine proteinases
These enzymes have been reported much less
frequently than cysteine proteinases in parasitic
protozoa. T. cruzi oligopeptidase B has
homology to members of the prolyl oligopeptidase
family (S9) of serine peptidases, and
appears to be indirectly involved in the
penetration of trypomastigotes, by activating
a factor which induces a transient increase of
cytosolic Ca 2 in the host cell. A T. brucei
enzyme with 70% identity to the T. cruzi enzyme
has no activity with typical prolyl oligopeptidase
substrates, but instead prefers substrates
characteristic of trypsin-like enzymes. Serine
oligopeptidases with similar specificity also
have been purified and partially characterized
from T. congolense and L. amazonensis. A prolyl
endopeptidase, reported to belong to the prolyl
oligopeptidase family, has been purified and
partially characterized from T. cruzi (Tc80), is
able to hydrolyze human collagens I and IV at
neutral pH, and its substrate specificity clearly
indicates that it is different from the oligopeptidases
B mentioned above. T. cruzi also contains
a lysosomal serine carboxypeptidase, belonging
to the S10 family, that is specific for C-terminal
hydrophobic amino acids.
P. falciparum serine and cysteine proteinases
participate together in the invasion
and/or rupture of red blood cells. The 75 kDa
serine protease is bound as an inactive precursor
to the membranes of schizonts and merozoites
by a glycosylphosphatidyl inositol (GPI)
anchor and is activated after release by a
specific phospholipase C. Since the purified
enzyme from merozoites is able to cleave two
major red blood cell proteins, glycophorin A
and Band III protein, it may be involved in the
formation of the parasitophorous vacuole from
the erythrocyte plasma membrane. PfSUB1, a
Ca 2 -dependent, membrane-bound, subtilisinlike
serine proteinase, processes the major
merozoite surface protein, MSP-1. The polypeptides
produced seem to be involved in the
receptor-ligand interactions which initiate the
invasion of the red blood cell. The precursor of
PfSUB1 seems to be a Type I integral membrane
protein. Both PfSUB1 and a second member of
the group, PfSUB2, are located in the dense
granules of the merozoites, and are processed
and secreted after completion of the red cell
invasion.
Metalloproteinases
These enzymes have been described in a
number of parasitic protozoa, but only those
present in promastigotes of Leishmania spp.
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA