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Fig. 1. Domain profiles of all PX-domain-containing proteins. Proteins were
selected from the SMART database (Schultz
et al., 2000 ), aligned, and unique proteins are shown (proteins
with >80% homology were excluded as redundant or orthologous). Where
appropriate, the human version is shown. Species abbreviations precede the
accession numbers; At, A. thaliana; Ca, C. albicans; Ce,
C.elegans; Dm, D. melanogaster; Hs, H.sapiens; Mm,
M.musculus; Nc, N.crassa; Os. O.satvia; Sc,
S.cerevisiae; Sp, S.pombe; Tb, T.brucei brucei; Um,
U.maydis. The residue at the at the equivalent position to Arg58 of
p40phox is shown as the standard single letter amino acid
code in each PX domain. Domains with a basic residue (R or K) at this position
are suggested to bind 3-phosphoinositides, whereas domains lacking a basic
residue are predicted to bind other phosphoinositide species (see text).
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-helix is not part of the core PX domain but was included in the
crystallised p40phox PX domain construct. MIL, membrane
interaction loop; PxxP, polyproline region. (B) Tertiary structure of the
overall topology of the p40phox PX domain bound to
PtdIns(3)P. The membrane interaction loop of the PX domain and the
acyl chains of the PtdIns(3)P would insert into a membrane located
towards the top of the diagram. (C) Details of the lipid-binding pocket shown
in (B). Key residues are illustrated to demonstrate how each one is positioned
to fulfil specific roles; Lys92 hydrogen bonds with the 1-phosphate, Tyr59
stacks against the inositol ring, Arg105 interacts with the 4- and 5-hydroyl
groups, Arg58 contacts the 3-phosphate and Arg57 faces away from the lipid
binding pocket, fulfilling a structural role in the domain.
PX can still facilitate docking, as it can be
localised through Vam3, albeit to a lower extent than the wild type). The
cargo vesicle docks to Vam7p through Vti1, and the Class C Vps complex also
joins. Assembly of this complex facilitates vesicle-vacuole fusion
(Sato et al., 2000