trans

178 PROTEIN METABOLISM
values ranging from 4.5 to 8.5. Genes encoding
proteasome subunits have been cloned from
most of these protozoa and in general exhibit
considerable homology to proteasome subunits
from higher animals. A proteasome-activating
protein, PA26 from T. brucei, polymerizes spontaneously
into a heptamer ring and is able to
activate the 20S proteasomes from the rat as
well as T. brucei. The active form of the proteasome
involves, in addition to the 20S catalytic
complex, two 19S activating complexes, made
up of a number of subunits, some of which possess
ATPase activity. A gene encoding a proteasome
S4 ATPase, a member of the 19S regulatory
complex, has been cloned from P. falciparum.
Recently a 26S proteasome with ATP-dependent
chymotrypsin-like activity has been purified
from T. cruzi. The purified complex consists of
about 30 proteins, with molecular masses ranging
from 25 to 110 kDa. The 26S proteasome
participates in protein turnover during differentiation
from trypomastigotes to amastigotes,
and its inhibition leads to the accumulation of
ubiquitinated proteins. In addition to the 20S
proteasome, E. histolytica trophozoites contain
an 11S proteinase, which seems to be a hexamer
of identical 60–65 kDa subunits, consisting of
two trimers which are still active under weakly
denaturing conditions.
Proteolytic enzymes of helminths
Schistosomes have a complex life cycle, involving
a snail as intermediate host. Proteinases are
involved in the active skin penetration of cercariae
liberated by the snail during their transformation
into schistosomula. A 28 kDa soluble
serine proteinase, with similarities to pancreatic
elastases, is released by the acetabular glands
during penetration. A similar antigenically
cross-reactive proteinase is GPI-anchored to
the tegument of mechanically transformed
schistosomulae. A metalloproteinase activity
associated with the surface of schistosomula
and adults also has been described. Adult schistosomes
feed on erythrocytes, which are lysed
in the gut; the hemoglobin is then digested. Two
immunodominant schistosome antigens, Sm31
and Sm32, are cysteine proteinases. Sm31 is a
cathepsin B-like cysteine proteinase, whereas
Sm32 is an asparaginyl endoproteinase, homologous
to the Canavalia ensiformis legumain, a
cysteine proteinase belonging to the C13 family,
quite different from the papain family. In addition,
two cathepsin L-like enzymes have been
recently described. L 1 is more closely related
to cruzipain, and L 2 is more closely related to
human cathepsin L. Fasciola hepatica also
has a snail as intermediate host, but its cercariae
infect man by ingestion and penetration
through the intestinal mucosa. A number of
cysteine proteinase genes have been amplified
from cDNAs of adult worms, including those
encoding a cathepsin L-like enzyme which
contains unusual 3-hydroxyproline residues
and a cathepsin B-like enzyme found in newly
excysted juveniles and in adults. A secreted serine
peptidase, with dipeptidylpeptidase activity,
has been identified in juveniles and adults.
Aspartic proteinases are also present. Recently,
a 28 kDa cruzipain-like cysteine proteinase has
been characterized in Paragonimus westermani;
the enzyme is located in the intestinal
epithelium, suggesting that it may be secreted
and could be involved in nutrient uptake.
Proteinases belonging to all catalytic classes
have been described in parasitic nematodes.
Ancylostoma caninum secretes 90 and 50 kDa
metalloproteinases important for skin penetration,
and contains a number of cathepsin
B- and L-like cysteine proteinases. Third and
fourth stage larvae of the ovine parasite
Haemonchus contortus contain a 46 kDa
metalloproteinase able to digest fibrinogen
and fibronectin, and a cysteine proteinase is
involved in the degradation of extracellular
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA