Biology Reference
In-Depth Information
reduced persistence, changing direction frequently. Additional support for a
defect in adhesion comes from an analysis of particle binding - the VASP null
cells exhibit significantly reduced binding of 1 mm latex beads when compared
with controls. This
lower binding,
surprisingly, does not
result
in a
corresponding phagocytosis defect (Han et al., 2002).
Integrins are major cell-surface receptors in higher eukaryotes and binding
to their extracellular ligand results in the recruitment of a number of
cytoplasmic proteins, including signalling and actin-binding proteins (Geiger
et al., 2001). One of these proteins is talin. Talin interacts with integrin
cytoplasmic tails, the focal adhesion protein vinculin and actin. Thus, it is
positioned to play a key role in linking external attachments to the actin
cytoskeleton (Calderwood et al., 2000). The N-terminus of talin harbours a
FERM domain, a region of shared homology between band 4.1, ezrin, radixin
and moesin, and the remaining portion of the molecule is predicted to be rod-
like. The rod domain harbours a binding site for vinculin and actin
(Hemmings et al., 1996). The FERM domain from a number of ERM
proteins has been crystallized and found to consist of three subdomains
arranged in a clover-leaf like shape (Hamada et al., 2000; Pearson et al., 2000).
The structure of the 'F3' FERM subdomain is remarkably similar to that of a
phosphotyrosine binding (PTB) domain and it interacts with the NpxY motif
of the cytoplasmic tail of b integrin (Calderwood et al., 1999; Garcı´ a-Alvarez
et al., 2003). The FERM domain of talin also binds to a specific
phosphoinositol phosphate kinase type 1g (PIPK1g) isoform that is targeted
to focal adhesion (Di Paolo et al., 2002; Ling et al., 2002). Stimulation of
PIPK1g activity by adhesion results in increased PIP(4,5)P
2
production that,
in turn, enhances the interaction of vinculin with actin and talin.
Dictyostelium expresses two distinct forms of talin - talinA and talinB
(Kreitmeier et al., 1995; Tsujioka et al., 1999) (Figure 2.1). These two proteins
are highly similar at their N- and C-termini, but are otherwise quite distinct.
Most notably, TalB has a villin headpiece at its C-terminus (Tsujioka et al.,
1999). TalA was identified in a search for actin binding proteins present in a
triple null mutant lacking three major actin binding proteins (gelation factor,
a-actinin and severin) by F-actin anity chromatography (Kreitmeier et al.,
1995). TalA is present in the cytosol, enriched at the leading edge of
chemotactic cells and present in filopodia (hence its original name, filopodin).
TalA has also recently been shown to be present in small dot-like structures on
the bottom of the cell that are reminiscent of focal contacts (Hibi et al., 2003).
The talA null mutant is defective in adhesion to the substratum as observed by
interference reflection contrast microscopy (IRM). The loss of substrate
binding results in decreased phagocytic activity and abnormal motility but
does not appear to affect the formation of filopodia (Niewo¨ hner et al., 1997;
Tuxworth et al., 2001). The talA null cells also exhibit a mild cytokinesis
defect and decreased membrane bending modulus (Niewo¨ hner et al., 1997;
