trans

GENETICS OF ANTIGENIC VARIATION 99
TABLE 5.1
Proteins encoded by expression-site-associated genes
ESAG kDa a Known or predicted properties of ESAG-encoded proteins b # c
1 36 Membrane glycoprotein 92
2 50 GPI-anchored surface glycoprotein 50
3 43 Secreted glycoprotein 360
4 139 Flagellar membrane-associated receptor adenyl/guanyl cyclase 500
5 46 Secreted glycoprotein 29
6 45 GPI-anchored transferrin receptor subunit 44
7 36 Non-GPI-anchored transferrin receptor partner of ESAG6 44
8 70 Predominantly nucleolar protein with RING zinc finger and 8
leucine-rich-repeat motifs
9 34 GPI-anchored surface glycoprotein 17
10 76 Biopterin transporter 30
11 41 GPI-anchored surface glycoprotein 26
a The indicated sizes are those of the precursor polypeptides, before removal of signal sequences and
addition of GPI and glycans.
b Predicted membrane glycoproteins have a candidate amino-terminal signal sequence and a potential
carboxy-terminal GPI signal or membrane-spanning hydrophobic domain(s). The cellular location of
proteins encoded by ESAGs 1, 2, 3, 5, 9 and 11 is unknown. ESAGs 3 and 5 have good candidate signal
sequences but are otherwise predicted to be soluble glycoproteins, destined for secretion or for
retention in the secretory pathway. Except for the heterodimeric transferrin receptor encoded by
ESAG6 and ESAG7, the functions of all of these predicted proteins are unknown. The adenyl cyclase
activity of ESAG4 has been demonstrated. Although co-transcribed with VSG, most ESAG-encoded
proteins appear to be present at less than 0.2% of the abundance of VSG.
c The last column (#) indicates the number of high-quality matches (p e 20 ) in a tBLASTn search of a
T. brucei database (European Bioinformatics Institute) containing about 100 000 random fragments
and complete genes, with a representative of each ESAG protein sequence. ESAGs 6 and 7 are
themselves very similar. For ESAG8, the value given is for a search using only the first 100 amino acids,
which distinguish ESAG8 from other leucine-rich-repeat proteins. Although the p cutoff value is
somewhat arbitrary, and the search does not distinguish functional genes from pseudogenes, some
trends are apparent. ESAG4 represents a large family of receptor adenyl cyclases that are not
ES-restricted and the abundance of ESAG3 suggests that there are also many non-ES loci. There must
also be non-ES-located copies of ESAG10, since this essential transporter is only found in some ESs,
as are ESAG9 and ESAG11. When a similar sequence database of Leishmania was searched, the only
comparable matches were 8 with ESAG4 and 20 with ESAG10.
Mechanisms of VSG expression
and switching
The expressed VSG can be changed by turning
off one ES and turning on another one (in situ
switching), by duplicative transposition (gene
conversion) of silent telomeric or non-telomeric
VSGs into the currently active ES (displacing
the previously expressed VSG), by telomere
exchange, or by telomere conversion/breakinduced
telomere repair (Figure 5.4A). There is
very tight control of VSG expression. Even by
putting different drug-selectable marker genes
into different ESs, a trypanosome cannot be
forced to simultaneously and stably co-express
two VSGs. Regulation of singularity does not
appear to be enforced at the level of VSG secretion
and coat stability, since trypanosomes
can be tricked into simultaneously expressing
two VSGs by inserting a second VSG in tandem
to the endogenous VSG in an active ES. There
must be cross-talk, at some level, between ESs,
MOLECULAR BIOLOGY