Biology Reference
In-Depth Information
and Globodera pallida, the NPA-encoding gene is transcribed in the cells of
the intestine, and in all life-cycle stages.
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The biochemical activity of
the NPAs remained a mystery until it was found by accident that ABA-1
and the other NPAs bind fatty acid and retinol (vitamin A).
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They
may therefore have a function similar to that of vertebrate serum albumin
in the bulk transport of sparingly soluble lipids within the worms. Lipid
binding by ABA-1 was discovered and analyzed using fluorescence-
based techniques, which not only allowed the range of lipids it can
bind to be investigated very quickly, but also showed that the lipid
binding site in the molecule is unusually apolar (
Figure 3.7
).
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So, there arose many reasons to understand the structure of ABA-1 and
its ilk, namely that (as a tandemly repetitive polyprotein) NPAs are a type
FIGURE 3.7
Lipid binding by ABA-1. Lipid binding activity was examined using
a fatty acid attached to a dansyl fluorescent group that is environment sensitive (dansyl
undecanoic acid, DAUDA, see inset). DAUDA has a low fluorescence emission in water, but
its emission increases dramatically and shifts to a shorter wavelength (blue shift) when it
enters a protein binding site. The degree of this shift is taken to indicate the degree of
apolarity of its new environment, and the blue shift with ABA-1 is unusually large. The
experiment shown compared a recombinant A-type repeat and a recombinant B-type
repeat, the characteristics of which were similar. It is not shown here, but when a natural
ligand, for example oleic acid, is added to the mixture of ABA-1 and DAUDA, it displaces
DAUDA from the binding site back into water and its fluorescence decreases. This allows
the rapid screening of natural lipids and other compounds for binding to these proteins.
