Biology Reference
In-Depth Information
to be virtually superimposable, despite diversity in their amino acid
sequences.
99,100
Nematodes also possess cytosol-confined FABPs that
appear to be similar to those found elsewhere in the animal phyla, but the
unusual subfamily typified by As-p18 appears to be confined to
nematodes (hence the name nemFABPs).
90
Significantly, only in nema-
todes do their genes encode a secretory leader peptide and the proteins
are secreted from the synthesizing cells.
89,90,98
And only in nematodes is
the mature polypeptide (after removal of the secretory signal peptide)
longer than in other FABPs, which was speculated to mean that the
mature proteins have extra loop regions unique to nemFABPs.
89
If their
structures do indeed have the anticipated extra features, then they may
have biological functions not seen in other phyla.
The structure of As-p18 was solved using both NMR and x-ray crys-
tallography.
101,102
As with ABA-1, the protein was produced in bacteria
and needed to be stripped of bacterial-derived lipids, then loaded
uniformly with a fatty acid (oleic) so as to produce a homogeneously
conformed preparation of protein. This revealed, as anticipated, that As-
p18 folds as a
-barrel surrounding a central cavity (
Figure 3.9
), the walls
of which are lined predominantly with apolar amino acid side chains.
This is consistent with a hydrophobic lipid binding pocket into which
a small lipid could partition. There are also some charged side chains
projecting into the cavity that could, as with other FABPs, be involved in
tethering a lipid's charged head group, although the orientation of lipid
within As-p18 remains to be determined. As with other FABPs, the
opening of the cavity is capped by two short antiparallel sections of
a
b
-helix. So, As-p18 does indeed conform to the general structure of
cytosolic lipid binding proteins of the FABP/CRBP/CRABP superfamily.
The atypical loop regions that had been predicted from earlier modeling
of As-p18 are present, but not where predicted.
89
As-p18 is instead found
to have an extended loop emerging from the opposite end of the molecule
from the presumptive portal region for entry and exit of ligands beneath
the pair of helices. Such a loop has not been found in any other member of
this protein superfamily, structures for which are now known from a wide
range of animal groups including vertebrates, insects, arachnids, and
cestodes. A short loop adjacent to the portal region is also longer than in
other FABPs. Thus, As-p18, and presumably other nemFABPs, does
indeed exhibit features that are so far confined to nematodes.
Another interesting feature of As-p18 that may provide a clue to its
function is the existence of a cluster of apolar amino acid side chains
projecting into solvent from the short helix adjacent to the portal
(
Figure 3.9
). A cluster similar to this is also seen in mammalian FABPs that
have been found to interact and exchange lipids with artificial membranes
by a direct contact, collisional mechanisms,
103
by which a lipid ligand can
transfer between a protein and a membrane without entry into the
