trans

AFRICAN TRYPANOSOMES 227
VSG
Procyclin
N N N
VSG
dimers
Amino terminal
domain
C-terminal domain
Repeat domain
SA 5 (Galβ1-4GlcNAc) 9 Polylactosamine side chains
EtN-PO 4
Manα1,2Manα1,6Manα1,4GlcNα1,6Inos-PO 4 -DAG
(/)Galα1,2Galα1-6Galα1-3
(/)Galα1-2
Manα1-2Manα1-6Manα1-4GlcNα1-6Inos-PO 4 -DAG
PO 4
EtN
NH 2
Manα1-6
Manα1-6
Manα1-3 Manβ1-4GlcNAcβ1-4GlcNAc
Manα1-3
FIGURE 10.1 A schematic representation of the major surface glycoconjugates of procyclic and metacyclic
T. brucei. VSG (variant surface glycoprotein) is the major component of the metacyclic form, each molecule
consisting of two GPI-anchored N-glycosylated monomers. The shaded ovals represent the protein component.
The surface of the procyclic form is densely covered with procyclins (PARPs or procyclic acidic repetitive proteins).
These are GPI-anchored polypeptides with polyanionic repeat domains (shaded in diagram). The anchor structures
and the N-linked oligosaccharide of procyclin are detailed below the schematic. DAG, diacylglycerol; EtN,
ethanolamine; GlcN, glucosamine; Inos, inositol; SA, sialic acid.
over 25 different surface proteins in procyclics.
However, procyclics contain a surface coat
composed largely of acidic glycoproteins called
the procyclins, also called PARPs or Procyclic
Acidic Repetitive Proteins (Figure 10.1). Each
cell expresses about 5 million copies of procyclins.
The procyclins form a dense glycocalyx
of GPI anchors, with the polyanionic polypeptide
repeat domains projecting above the
membrane. There are two families of procyclins:
the EP-procyclins have glutamate–proline
repeats while the GPEET-procyclins bear
glycine–proline–glutamate–glutamate–threonine
repeat sequences. Some members of the
EP-procyclin family are N-glycosylated adjacent
to the repeat domain. An unusual feature
of this glycosylation site is that only a single
type of N-glycan is found, namely a Man 5 Glc
NAc 2 oligosaccharide. Properly glycosylated
forms of EP are required for concanavalin
A-induction of a unique form of cell death in
T. brucei, defined as proto-apoptosis. The GPI
anchors are modified with unusual branched
polylactosamine {Gal(1,4)GlcNAc} glycans.
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA