Paxillin is a protein that in humans is encoded by the PXN gene. Paxillin is expressed at focal adhesions of non-striated cells and at costameres of striated muscle cells, and it functions to adhere cells to the extracellular matrix. Mutations in PXN as well as abnormal expression of paxillin protein has been implicated in the progression of various cancers.
Human paxillin is 64.5 kDa in molecular weight and 591 amino acids in length. [5]
The C-terminal region of paxillin is composed of four tandem double zinc finger LIM domains that are cysteine/histidine-rich with conserved repeats; these serve as binding sites for the protein tyrosine phosphatase-PEST, [6] tubulin [7] and serves as the targeting motif for focal adhesions. [8]
The N-terminal region of paxillin has five highly conserved leucine-rich sequences termed LD motifs, which mediate several interactions, including that with pp125FAK and vinculin. [9] [10] The LD motifs are predicted to form amphipathic alpha helices, with each leucine residue positioned on one face of the alpha helix to form a hydrophobic protein-binding interface. The N-terminal region also has a proline-rich domain that has potential for Src-SH3 binding. Three N-terminal YXXP motifs may serve as binding sites for talin or v-Crk SH2. [11] [12]
Paxillin is a signal transduction adaptor protein discovered in 1990 in the laboratory of Keith Burridge [13] The C-terminal region of paxillin contains four LIM domains that target paxillin to focal adhesions. It is presumed through a direct association with the cytoplasmic tail of beta-integrin. The N-terminal region of paxillin is rich in protein–protein interaction sites. The proteins that bind to paxillin are diverse and include protein tyrosine kinases, such as Src and focal adhesion kinase (FAK), structural proteins, such as vinculin and actopaxin, and regulators of actin organization, such as COOL/PIX and PKL/GIT. Paxillin is tyrosine-phosphorylated by FAK and Src upon integrin engagement or growth factor stimulation, [14] creating binding sites for the adapter protein Crk.
In striated muscle cells, paxillin is important in costamerogenesis, or the formation of costameres, which are specialized focal adhesion-like structures in muscle cells that tether Z-disc structures across the sarcolemma to the extracellular matrix. The current working model of costamerogenesis is that in cultured, undifferentiated myoblasts, alpha-5 integrin, vinculin and paxillin are in complex and located primarily at focal adhesions. During early differentiation, premyofibril formation through sarcomerogenesis occurs, and premyofibrils assemble at structures that are typical of focal adhesions in non-muscle cells; a similar phenomenon is observed in cultured cardiomyocytes. [15] Premyofibrils become nascent myofibrils, which progressively align to form mature myofibrils and nascent costamere structures appear. Costameric proteins redistribute to form mature costameres. [16] While the precise functions of paxillin in this process are still being unveiled, studies investigating binding partners of paxillin have provided mechanistic understanding of its function. The proline-rich region of paxillin specifically binds to the second SH3 domain of ponsin, which occurs after the onset of the myogenic differentiation and with expression restricted to costameres. [17] We also know that the binding of paxillin to focal adhesion kinase (FAK) is critical for directing paxillin function. The phosphorylation of FAK at serine-910 regulates the interaction of FAK with paxillin, and controls the stability of paxillin at costameres in cardiomyocytes, with phosphorylation reducing the half-life of paxillin. [18] This is important to understand because the stability of the FAK-paxillin interaction is likely inversely related to the stability of the vinculin-paxillin interaction, which would likely indicate the strength of the costamere interaction as well as sarcomere reorganization; processes which have been linked to dilated cardiomyopathy. [19] Additional studies have shown that paxillin itself is phosphorylated, and this participates in hypertrophic signaling pathways in cardiomyocytes. Treatment of cardiomyocytes with the hypertrophic agonist, phenylephrine stimulated a rapid increase in tyrosine phosphorylation paxillin, which was mediated by protein tyrosine kinases. [20]
The structural reorganization of paxillin in cardiomyocytes has also been detected in mouse models of dilated cardiomyopathy. In a mouse model of tropomodulin overexpression, paxillin distribution was revamped coordinate with increased phosphorylation and cleavage of paxillin. [21] Similarly, paxillin was shown to have altered localization in cardiomyocytes from transgenic mice expressing a constitutively-active rac1. [22] These data show that alterations in costameric organization, in part via paxillin redistribution, may be a pathogenic mechanism in dilated cardiomyopathy. In addition, in mice subjected to pressure overload-induced cardiac hypertrophy, inducing hypertrophic cardiomyopathy, paxillin expression levels increased, suggesting a role for paxillin in both types of cardiomyopathy. [23]
Paxillin has been shown to have a clinically-significant role in patients with several cancer types. Enhanced expression of paxillin has been detected in premalignant areas of hyperplasia, squamous metaplasia and goblet cell metaplasia, as well as dysplastic lesions and carcinoma in high-risk patients with lung adenocarcinoma. [24] Mutations in PXN have been associated with enhanced tumor growth, cell proliferation, and invasion in lung cancer tissues. [25]
During tumor transformation, a consistent finding is that paxillin protein is recruited and phosphorylated. [26] Paxillin plays a role in the MET tyrosine kinase signaling pathway, which is upregulated in many cancers. [27]
Paxillin has been shown to interact with:
In mammalian cells, vinculin is a membrane-cytoskeletal protein in focal adhesion plaques that is involved in linkage of integrin adhesion molecules to the actin cytoskeleton. Vinculin is a cytoskeletal protein associated with cell-cell and cell-matrix junctions, where it is thought to function as one of several interacting proteins involved in anchoring F-actin to the membrane.
Growth factor receptor-bound protein 7, also known as GRB7, is a protein that in humans is encoded by the GRB7 gene.
Growth factor receptor-bound protein 2, also known as Grb2, is an adaptor protein involved in signal transduction/cell communication. In humans, the GRB2 protein is encoded by the GRB2 gene.
Integrin-linked kinase is an enzyme that in humans is encoded by the ILK gene involved with integrin-mediated signal transduction. Mutations in ILK are associated with cardiomyopathies. It is a 59kDa protein originally identified in a yeast-two hybrid screen with integrin β1 as the bait protein. Since its discovery, ILK has been associated with multiple cellular functions including cell migration, proliferation, and adhesion.
Integrin beta-1 (ITGB1), also known as CD29, is a cell surface receptor that in humans is encoded by the ITGB1 gene. This integrin associates with integrin alpha 1 and integrin alpha 2 to form integrin complexes which function as collagen receptors. It also forms dimers with integrin alpha 3 to form integrin receptors for netrin 1 and reelin. These and other integrin beta 1 complexes have been historically known as very late activation (VLA) antigens.
Mitogen-activated protein kinase 14, also called p38-α, is an enzyme that in humans is encoded by the MAPK14 gene.
PTK2 protein tyrosine kinase 2 (PTK2), also known as focal adhesion kinase (FAK), is a protein that, in humans, is encoded by the PTK2 gene. PTK2 is a focal adhesion-associated protein kinase involved in cellular adhesion and spreading processes. It has been shown that when FAK was blocked, breast cancer cells became less metastatic due to decreased mobility.
Crk-like protein is a protein that in humans is encoded by the CRKL gene.
Transforming growth factor beta-1-induced transcript 1 protein is a protein that in humans is encoded by the TGFB1I1 gene. Often put together with and studied alongside TGFB1I1 is the mouse homologue HIC-5. As the name suggests, TGFB1I1 is an induced form of the larger family of TGFB1. Studies suggest TGFB1I1 plays a role in processes of cell growth, proliferation, migration, differentiation and senescence. TGFB1I1 is most localized at focal adhesion complexes of cells, although it may be found active in the cytosol, nucleus and cell membrane as well.
Protein tyrosine kinase 2 beta is an enzyme that in humans is encoded by the PTK2B gene.
Breast cancer anti-estrogen resistance protein 1 is a protein that in humans is encoded by the BCAR1 gene.
Rho guanine nucleotide exchange factor 7 is a protein that in humans is encoded by the ARHGEF7 gene.
Cytoplasmic protein NCK2 is a protein that in humans is encoded by the NCK2 gene.
FYN binding protein (FYB-120/130), also known as FYB, ADAP, and SLAP-130 is a protein that is encoded by the FYB gene in humans. The protein is expressed in T cells, monocytes, mast cells, macrophages, NK cells, but not B cells. FYB is a multifunctional protein involved in post-activation T cell signaling, lymphocyte cytokine production, cell adhesion, and actin remodeling.
Alpha-parvin is a protein that in humans is encoded by the PARVA gene.
Leupaxin is a protein that in humans is encoded by the LPXN gene.
Talin-1 is a protein that in humans is encoded by the TLN1 gene. Talin-1 is ubiquitously expressed, and is localized to costamere structures in cardiac and skeletal muscle cells, and to focal adhesions in smooth muscle and non-muscle cells. Talin-1 functions to mediate cell-cell adhesion via the linkage of integrins to the actin cytoskeleton and in the activation of integrins. Altered expression of talin-1 has been observed in patients with heart failure, however no mutations in TLN1 have been linked with specific diseases.
Keith Burridge is a British researcher and Kenan distinguished Professor at the University of North Carolina at Chapel Hill. His research on focal adhesions includes the discovery of many adhesion proteins including vinculin, talin and paxillin, and ranks him in top 1% of the most cited scientist in the field of molecular biology and genetics. Burridge has published more than 200 peer reviewed articles.
In structural and cell biology, the focal adhesion targeting domain is a conserved protein domain that was first identified in focal adhesion kinase (FAK), also known as PTK2 protein tyrosine kinase 2 (PTK2).
Talin 2 is a protein in humans that is encoded by the TLN2 gene. It belongs to the talin protein family. This gene encodes a protein related to talin 1, a cytoskeletal protein that plays a significant role in the assembly of actin filaments. Talin-2 is expressed at high levels in cardiac muscle and functions to provide linkages between the extracellular matrix and actin cytoskeleton at costamere structures to transduce force laterally.
