Biophysical studies of membrane proteins/peptides. Interaction with ...

The signalling functions of PI(4,5)P
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are performed via interactions with signalling
proteins. Several proteins with known actin regulatory properties have been shown to
interact with PI(4,5)P . N-WASP, a protein responsible for the stimulation of actin
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nucleation is activated by binding to PI(4,5)P
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through a short polybasic domain
(Papayannopoulos et al., 2005). Overall, it seems that free (non protein-bound)
PI(4,5)P in the plasma membrane acts as a signal for anchoring of actin to the
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membrane (McLaughlin et al., 2002). Several proteins related to the exocytosis and
clathrin mediated endocytosis mechanisms have already been shown to specifically bind
PI(4,5)P , most notably epsin, a protein with membrane remodelling properties that was
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shown to bind lipid membranes through interaction with PI(4,5)P (Ford et al., 2002).
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The question of how can a single lipid species regulate so many different functions
in the cell with an apparent spatial resolution is still open. It has been hypothesized that
the distribution of PI(4,5)P in the plasma membrane is non-uniform and that pools of
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spatially confined PI(4,5)P must exist (Martin, 2001). Several authors detected
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evidence of cholesterol dependent localization of PI(4,5)P
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within domains in the
plasma membrane (Pike and Miller, 1998; Liu et al., 1998; Laux et al., 2000; Botelho et
al., 2000; Kwik et al., 2003; Epand et al., 2004; Epand et al., 2005; Gokhale et al.,
2005). These observations led to the conclusion that PI(4,5)P were preferentially bound
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to raft domains. However, some authors still disagree with the hypothesis of PI(4,5)P
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enrichment in rafts (van Rheenen et al., 2005).
Spontaneous partition of PI(4,5)P to lipid rafts is highly unlikely, as this lipid
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almost always presents polyunsaturated acyl-chains which are not expected to favour
incorporation in the more rigid environment found in rafts (McLaughlin et al., 2002).
One possible explanation for the detection of PI(4,5)P in these structures would be its
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local synthesis. However, this cannot account for the levels detected, as diffusion is
expected to occur at faster rates than synthesis (Ghambir et al., 2004). The most likely
mechanism for PI(4,5)P concentration in lateral domains is that some proteins with
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favoured partition to rafts can act as buffers, binding and passively concentrating these
lipids. A family of proteins (GMC proteins) have been identified that are likely to play
this role. GAP43, myristoylated alanine-rich C kinase substrate (MARCKS), CAP23,
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