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Gelsolin and Phospholipids

In 1984, Lassing and Linberg, raised a few eyebrows by their discovery that the well documented protein profilin, bound specifically to phosphatidylinositol 4, 5-bisphosphate, (PIP2) a highly charged phospholipid found at the cytoplasmic leaflet of the plasma-membrane (Lassing & Linberg, 1985) (Right).  This report was quickly followed by others who found that not only did other ABPs bind these lipids, but that in doing so, their actin-binding properties were modulated (e.g. Gelsolin see below).  The gelsolin cap at the end of the actin filament is very tight but it was shown that PIP2 could dissociate the complex so that actin monomers can now leave the barbed end of the filament (Janmey et al, 1987a, Janmey et al, 1987b).

 

PIP2

Gelsolin is present in inactivated cells on the barbed end of the filament, preventing further polymerization.  Upon stimulation, gelsolin is removed from the filament end by the production of PIP2.  Further actin polymerization can now take place at the fast growing end, driving the cell forwards possibly by the Brownian ratchet mechanism. PIP2 also leads to the dissociation of the profilin :actin, and the cofilin:actin complexes so that more actin would be available for polymerization.
PIP2 is known to bind many actin-binding and other proteins and to modulate their activity.
References:-

Janmey, P. A., Chaponnier, C., Lind, S. E., Zaner, K. S., Stossel, T. P. & Yin, H. L. (1985) Interactions of gelsolin and gelsolin-actin complexes with actin. Effects of calcium on actin nucleation, filament severing and end blocking., Biochemstry. 24, 3714-3723.

Janmey, P. A. & Stossel, T. P. (1987a) Modulation of gelsolin function by phosphatidylinositol 4,5-bisphosphate, Nature. 325, 362-364.

Janmey, P. A., Iida, K., Yin, H. L. & Stossel, T. P. (1987b) Polyphosphoinositide micelles and polyphosphoinoositide-containing vesicles dissociate endogenous gelsolin-actin complexes and promote actin assembly from the fast-growing end of actin filaments blocked by gelsolin, J. Biol. Chem. 262, 12228-12236.

Janmey, P. A., Lamb, J., Allen, P. G. & Matsudaira, P. T. (1992) Phosphoinositide-binding peptides derived from the sequences of gelsolin and villin, J. Biol. Chem. 267, 11818-11823.

Lassing, I. & Lindberg, U. (1985) Specific interaction between phosphatidylinositol 4,5-bisphosphate and profilactin, Nature. 314, 472-474.

 
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