Profilin

Profilin

updated 29/4/02

A low molecular weight actin binding protein which seems to have been found in every eukaryotic species where it has been looked for. The protein was identified after the discovery of a non-polymerised form of actin from Thyone sperm (Tilney ). This complex was found to contain actin, and a 16kDa protein and was named "profilamentous actin" (Tilney, ) later abbreviated to "profilactin" (Tilney, 1976b). However, although it is very likely that the 16 kDa protein is profilin, I do not believe that this has ever been shown. The name "profilin" was coined by Carlson from the previous term "profilactin". The amino-acid sequence of the profilins are not very highly conserved across the many phyla from those determined (protozoa, mammalian, yeast, insect, echinoderm, plant and even a virus), yet despite this the structure is conserved (Schutt et al, 1993; Vinson et al, 1993; Federov et al, 1994; Federov et al, 1997). Possibly connected with the sequence diversity, the ubiquity and abundance of profilins in eukaryotes is the fact that profilins are major allergens. Profilin has an unusual property in its ability to bind poly-L-proline, and although the physiological significance of this accidental finding (Tanaka & Shibata, 1985) is in itself very important, this has provided an easy way to purify the protein (Kaiser et al, 1989; Janmey, 1991). Profilin binds to poly-L-proline as its actual function is to bind proline rich regions in a number of profilin binding proteins. Not only does profilin bind a series of partners through the proline -rich binding domain but it also binds phosphatidylinositol 4, 5 bisphosphate (Lassing & Lindberg, 1985). (This discovery was the first of a very long list of proteins that are now known to bind PIP₂). The specificity of the interaction has been determined by others (Machesky et al, 1992). The phosphoinositides dissociates the profilactin complex but the situation is very much more interesting than this as profilin can shield PIP₂ against hydrolysis by phospholipase C g but not when the lipase has been activated by phosphorylation (Goldschmidt-Clermont et al, 1991 ) bi and its low molecular weight ( about 15kd), profilin retains its ability to bind:-actin, poly-proline, and phosphoinositides. This ability to bind phosphoinositides is most intriguing and it has even been suggested to be the primary role of the protein in cells, since phosphoinositides provide a very important cell signalling cascade, transducing messages from outside to inside the cell.

The role of profilin in actin polymerization.

Partly because of how it was first discovered and on its effects on the polymerization of pure actin, profilin has been described primarily as an actin monomer sequestering protein. However, it is now appreciated that the role of profilin is much more subtle than being merely an actin sequestering protein. In some situations profilin encourages the polymerization of actin onto the barbed ends of actin filaments.

Intracellular localisation of profilin.

In fibroblasts profilin has been found to be concentrated at the leading edge (Bub et al, 1992). Profilin is localised too to sites where PIP₂ is enriched in plant hair cells (Braun et al, 1999), at the leading edge in the amoeba Acanthamoeba (Bubb et al, 1998)

The role of profilin in cytokinesis.

Profilin has been found to be necessary for complete cytokinesis in a number of organisms such as Schizosaccharomyces pombe (Balasubramanian et al, 994), Saccharomyces cerevisiae (Chang et al, 1997), Dictyostelium discoidium (Haugwitz et al, 1994)

The self-association of profilin?

Some reports suggest that profilin can self-associate forming oligomers (Babich et al, 1996)

Related topics within this site:-

Profilin binding proteins	Membrane Cytoskeleton Interactions
Dictyostelium ABPs	Arp2/3 complex

Other Related Web-sites:-

References:-

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