Profilin
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2008-06-17T16:02:34Z
DumZiBoT
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robot Adding: [[it:Profilina]]
[[Image:Profilin actin complex.png|thumb|200px|Profilin (blue) in complex with [[actin]] (green). ([[Protein_Data_Bank|PDB]] code: 1BTF)]]
'''Profilin''' is an [[actin-binding protein]] involved in the dynamic turnover and restructuring of the [[actin]] [[cytoskeleton]]. It is found in all [[Eukaryote|eukaryotic]] organisms in most [[cell (biology)|cells]]. Profilin is important for spatially and temporally controlled growth of [[actin]] microfilaments, which is an essential process in cellular locomotion and cell shape changes. This restructuring of the actin cytoskeleton is essential for processes such as [[organogenesis|organ development]], [[wound healing]], and the hunting down of infectious intruders by cells of the [[immune system]].
Profilin also binds sequences rich in the [[amino acid]] [[proline]] in diverse proteins. While most profilin in the cell is bound to actin, profilins have over 50 different binding partners. Many of those are related to actin regulation, but profilin also seems to be involved in activities in the [[cell nucleus|nucleus]] such as [[mRNA]] [[splicing (genetics)|splicing]].
Profilin binds some variants of [[cell membrane|membrane]] [[phospholipid]]s ([[Phosphatidylinositol (4,5)-bisphosphate]] and [[Inositol triphosphate]]). The function of this interaction is the sequestration of profilin in an "inactive" form, from where it can be released by action of the enzyme [[phospholipase C]].
Profilin is the major [[allergen]] present in [[birch]], grass, and other [[pollen]].
==Profilin sources and distribution==
Profilins are proteins of [[molecular weight]]s of roughly 14 - 16 kDa. They are present as single genes in yeast, insects, and worms, and as multiple genes in many other organisms including plants. In [[mammal]]ian cells, four profilin [[protein isoform|isoforms]] have been discovered; profilin-I is expressed in most [[Biological tissue|tissues]] while profilin-II is predominant in [[brain]] and [[kidney]].<ref name="Witke">Witke, W., Podtelejnikov, A., Di Nardo, A., Sutherland, J., Gurniak, C., Dotti, C., and M. Mann (1998) In Mouse Brain Profilin I and Profilin II Associate With Regulators of the Endocytic Pathway and Actin Assembly. The [[EMBO Journal]] 17(4): 967-976 {{Entrez Pubmed|9463375}}</ref>
==Profilin in the regulation of actin dynamics==
Profilin enhances actin growth in two ways:
* Profilin binds to monomeric actin thereby occupying an actin-actin contact site; in effect, profilin sequesters actin from the pool of polymerizable actin monomers. However, profilin also catalyzes the exchange of actin-bound [[Adenosine diphosphate|ADP]] to [[Adenosine triphosphate|ATP]] thereby converting poorly polymerizing ADP-actin monomers into readily polymerizing ATP-actin monomers. On top of that, profilin has a higher affinity for ATP- than for ADP-actin monomers. Thus in a mixture of actin, profilin, and nucleotides (ADP and ATP), actin will polymerize to a certain extent, which may be estimated by the [[law of mass action]].
* Profilin-actin complexes are fed into growing actin polymers by proteins such as [[formin]], [[Wiskott-Aldrich syndrome protein|WASP]] and VASP (that contain proline-rich FH2-domains). This mode of stimulated actin polymerization is much faster than unaided polymerization. Profilin is essential for this mode of polymerization because it recruits the actin monomers to the proline-rich proteins.
Profilin is one of the most abundant actin monomer binders, but proteins such as [[CAP (protein)|CAP]] and (in mammals) [[thymosin]]β4 have some functional overlaps with profilin. In contrast, ADF/[[cofilin]] has some properties that antagonize profilin action.
==History of profilin discovery==
Profilin was first described by Uno Lindberg and co-workers in the early 1970's as the first actin monomer binding protein.<ref name="Carlsson">Carlsson L, Nyström LE, Sundkvist I, Markey F, Lindberg U. (1977) Actin polymerizability is influenced by profilin, a low molecular weight protein in non-muscle cells. J. Mol. Biol. 115:465-483 {{Entrez Pubmed|563468}}</ref> It followed the realization that not only muscle, but also non-muscle cells, contained high concentrations of actin, albeit in part in an unpolymerized form. Profilin was then believed to sequester actin monomers (keep them in a ''pro-filamentous'' form), and release them upon a signal to make them accessible for fast actin polymer growth.
==Genes==
* [[PFN1]], [[PFN2]], [[PFN3]], [[PFN4]]
==References==
<references/>
==External links==
* {{MeshName|Profilins}}
{{Cytoskeletal Proteins}}
[[Category:Proteins]]
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