Parvin: Difference between revisions

===Focal adhesions and cell migration===

Alpha-parvin possesses 6 putative proline-directed serine/threonine phosphorylation targets (residues 4, 8, 14, 16, 19, 61), of which serines 4 and 8 were shown to be the most important. Phosphorylation of alpha-parvin at serines 4 and 8 is correlated with the tightly regulated process of FA turnover during cell migration. Firstly, phosphorylation of these residues by cyclin B1/cdc2 is observed in the context of mitosis, whereby it contributes to FA disassembly required for cell-rounding prior to cell division, suggesting it may cause a similar effect during cell migration.<ref>PMID: 11931650</ref> Indeed, in migrating cells these residues are observed to be phosphorylated<ref>PMID: 15353548</ref>, likely as a result of MAP kinase<ref>PMID: 14636584</ref> and/or [[PI3K]]<ref>PMID: 12960424</ref>. Secondly, phosphomimetic mutations of serines 4 and 8 to aspartates result in faster migration and spreading, while mutations preventing phosphorylation impair these processes.<ref>PMID: 15353548</ref> Finally, as has already been mentioned in the introduction, knock-out mice phenotype (embryonic lethality due to severe cardiovascular defects) suggests alpha-parvin deficiency results in impaired directional migration of endothelial cells during embryonic development, heart development in particular.<ref>PMID: 19798050</ref> The macroscopic effects of alpha-parvin phosphorylation likely result from the altered affinity for its binding partners. So far it has been demonstrated that phosphorylation at serines 4 and 8 affects binding of alpha-parvin to TESK1 and CdGAP. When TESK1 is bound to alpha-parvin it is prevented from severing actin fibres. TESK1 is thought to be released from inhibition upon alpha-parvin phosphorylation and this can contribute to the decomposition of actin fibres and FA disassembly.<ref>PMID: 15817463</ref> CdGAP, on the other hand, is involved in the regulation of small GTPase signalling, which accounts for changes in cytoskeletal contractability during cell migration.<ref>PMID: 16860736</ref> Evidence suggesting that also the interaction with ILK is affected by phosphorylation is not strong<ref>PMID: 15872073</ref><ref>PMID: 12960424</ref>, but it is likely that this or yet other binding partners bind in the phosphorylation-dependent manner.

===Domain composition===

Alpha-parvin's structure can be divided into four regions: 1) N-terminal flexible domain (residues 1-96), 2) N-terminal CH domain (97-200 according to SMART<ref>PMID: 9600884</ref>), 3) linker region (201-241) and 4) C-terminal CH domain (242-372 identified by limited subtilisin proteolysis<ref>PMID: 18940607</ref>). Most interactions of alpha-parvin are mapped to the C-terminal CH domain, but CdGAP and perhaps alphaPIX or other, yet unknown partners, interact with the N-terminal flexibile domain. This flexible domain seems to lack a well defined 3D structure and can therefore be classified as a putative [[Intrinsically Disordered Protein|intrinsically disordered]] region. The interactions of this regions with the binding partners are therefore likely to be characterized by relatively low affinity, but high affinity nonetheless.<ref>PMID: 19265676</ref> The abovementioned phosphorylation sites (serines 4 and 8) involved in focal adhesion regulation are located in this segment, which makes them so called disorder-enhanced phosphorylation sites.<ref>PMID: 14960716</ref><ref>PMID: 18388127</ref>

===C-terminal CH domain===

The calponin-homology (CH) domains are helical structural units around 100 amino acids long. They comprise at least four helices, three of them forming a helical bundle. CH domains usually comprise elements of big multidomain proteins and are present either in singlet<ref>PMID: 19459066</ref> or duplex/tandem arrangement.<ref>PMID: 19565353</ref> The tandem arrangement of CH domains is often associated with F-actin binding<ref>PMID: 9708889</ref><ref>PMID: 18952167</ref> (and is thus called actin-binding domain or ABD), but generally CH domains seem to be characterized by functional plasticity and ability to bind various structural motifs.<ref>PMID: 11911887</ref> In the case of alpha-parvin, the interactions of CH domains with both F-actin and other partners (paxillin, ILK) are observed. The interactions with paxillin and ILK are mediated by a single CH domain, the C-terminal one. This domain has attracted most attention. While no full-length structure of alpha-parvin has been solved to date, the structure of the C-terminal CH domain, on its own<ref>PMID: 18940607</ref> and in complexes (with paxillin<ref>PMID: 18940607</ref><ref>PMID: 18508764</ref> and the pseudokinase domain of ILK<ref>PMID: 20005845</ref>) are available.  

The most diverged fragments in the C-terminal CH domain of alpha-parvin correspond to 1) an <scene name='User:Marcin_Jozef_Suskiewicz/Sandbox_Parvin//Parvin_overlap/3'>additional helix</scene> (so called N-terminal linker helix; it is labelled αN or helix N) located at the N-terminal end of the domain and not observed in any other CH domain and 2) a <scene name='User:Marcin_Jozef_Suskiewicz/Sandbox_Parvin//Parvin_overlap/5'>long loop</scene> between two helices (αC and αE). One could ask whether the N-terminal linker helix is an integral part of the C-terminal CH domain or just a part of a linker region between the two CH domains. The fact that it interacts strongly with the core helices by means of both electrostatic (<scene name='Alpha-parvin/Parvin_overlap/1'>residues D248, D251 and D255 with K355 and R359</scene>) and hydrophobic (<scene name='Alpha-parvin/Parvin_overlap/4'>residues F250, L253 and F254 with L354, K355, L358, R359, K260 and L261</scene>) interactions suggests that it is indeed integral to the domain. The long loop mentioned above contains a <scene name='User:Marcin_Jozef_Suskiewicz/Sandbox_Parvin//Parvin_overlap/6'>3-amino acid insertion</scene> (313-315) relative to other known CH domains and differs in conformation between different structures of alpha-parvin, suggesting it is relatively flexible.<ref>PMID: 18940607</ref> Interestingly, these two regions, the N-terminal linker helix and the long loop, are involved in binding of alpha-parvin to its binding partners, paxillin and ILK respectively.

===Paxillin binding===

The <scene name='Alpha-parvin/Parvin/2'>scene on the right</scene> shows the superimposition of the three conformations that alpha-parvin adopts when bound to paxillin LD motifs, LD1 ([[2vzd]]), LD2 ([[2vzg]]) and LD4 ([[2vzi]]) respectively. These three LD motifs differ in sequence, but they are all helical. Surprisingly, the orientation of LD1 binding is reversed compared to that of LD2 and LD4. One of the LD motifs (LD1) is shown in the scene, represented by a blue helix. LD2 and LD4 are not shown, but they bind in the same location. As you can see, all three peptides, despite different sequences and different binding orientations, induce a very similar conformation of alpha-parvin, as represented by a very good alignment of the three alpha-parvin structures coloured differently. In particular, residues 248 to 264, which experience conformational change upon binding, are similar in all complexes with RMSD values of 0.28 Å (LD1 versus LD2), 0.23 Å (LD1 versus LD4), and 0.15 Å (LD2 versus LD4) in 16 equivalent C^α positions.<ref>PMID: 18940607</ref>  

===ILK binding===

The <scene name='Alpha-parvin/Ilk_parvin/1'>scene on the left</scene> shows the complex ([[3kmw]]) of the kinase domain of integrin-linked kinase (ILK, red) bound to the C-terminal CH domain of alpha-parvin (blue). One can also see the molecule of ATP (green) and the space-fill representation of the magnesium atom (white). When we <scene name='Alpha-parvin/Ilk_parvin/2'>turn the structure</scene> so that the N-terminal helix (now orange) of the CH domain of alpha-parvin is pointing up, we can see that unlike paxillin LD motifs, ILK kinase domain does not bind to the N-terminal region of the CH domain, but rather near the <scene name='Alpha-parvin/Ilk_parvin/3'>long loop</scene> between helices αC and αE. Other parts of the CH domain are also involved in binding, leading to a high interface area (around 1900 Ų) characteristic of high-affinity complexes.<ref>PMID:20005845</ref> Interestingly, ILK, which was recently proved to lack kinase activity<ref>PMID:20005845</ref><ref>PMID: 20033063</ref>, binds alpha-parvin analogously to the way in which kinases bind their substrates, i.e. with its pseudoactive site. The binding is not dependent on the presence of ATP. On the ILK's side the binding is mediated primarily by <scene name='Alpha-parvin/Ilk_parvin/6'>one of the helices</scene> (αG) and a <scene name='Alpha-parvin/Ilk_parvin/5'>part of the activation loop</scene>. The complex formation is particularly dependent on <scene name='Alpha-parvin/Ilk_parvin/7'>methionine 402 and lysine 403</scene> in αG of ILK - if these two residues are mutated to alanines, the complex formation is completely abolished. These residues are involved in many interactions with alpha-parvin (one of them, a hydrogen bond to asparagine 280, is shown) or water molecules (one of them shown as a pink dot).

==References==

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