Biology Reference
In-Depth Information
have identical amino acid sequences, some slightly different, and at least
one exists that is only about 49% identical (
Figure 3.6
).
87,88
ABA-1 is the founding member of the nematode polyprotein allergens
(NPAs), and its formal systematic name is As-NPA-1 for the A. suum form,
and Al-NPA-1 for that of A. lumbricoides.
82
The individual repeated units
are designated by a terminal letter such as As-NPA-1A. NPAs have been
found widely in many parasitic and free-living species of nematode
within the Secernentean nematodes at least (which includes most of the
major nematode parasites of humans and plants except for groups such as
Trichinella, Trichuris, and Capillaria). Nematodes vary greatly in the
quantities of NPAs they produce, ascaridids, especially Ascaris, being
particularly notable in the amounts they make. In both Ascaris, C. elegans,
FIGURE 3.6
Organization of the precursor polyprotein of the ABA-1 allergen of
Ascaris lumbricoides
and
Ascaris suum
and amino acid sequence heterogeneity between
units. Upper panel: The polyprotein comprises a tandemly repeated array, each unit of
which encodes an ABA-1-like protein of about 14.4 kDa in size. The polyprotein is cleaved
post-translationally at regularly-spaced proteinase cleavage sites (indicated by *) into
multiple copies of ABA-1 proteins. At the carboxy terminal of the pre-processed polyprotein
there is a short extension that is removed (Ext). The complete sequence of the polyprotein
remains to be determined, and is still incomplete at the N-terminal end (as indicated by ???),
but is likely to comprise a short secretory signal peptide, as found in other species.
83
Those
units that have amino acid sequences that are identical or closely similar are designated
units A1 to 4, and one designated B1 that is only about 49% identical to the A-types. Lower
panel: Alignment of an A1- and a B1-type unit. Identical amino acids are indicated by an
asterisk (*) and those that are similar in biophysical properties are indicated by a colon (:).
The single tryptophan (W; single letter amino acid code), and the two cysteines (C) that are
cross-linked internally in the protein, are absolutely conserved in NPAs from all species of
nematode for which the sequences are known, and are double underlined in the sequences.
The terminal runs of four arginines (RRRR) are where the precursor protein is cut and the
arginines then removed. Regardless of the substantial differences in amino acid sequences,
the A1 and B1 types have very similar lipid binding properties (see Figure 3.7 and
87
).
