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The lamellipodium is a cytoskeletal protein actin projection on the leading edge of the cell. It contains a quasi-two-dimensional actin mesh; the whole structure propels the cell across a substrate. Within the lamellipodia are ribs of actin called microspikes, which, when they spread beyond the lamellipodium frontier, are called filopodia. The lamellipodium is born of actin nucleation in the plasma membrane of the cell and is the primary area of actin incorporation or microfilament formation of the cell.
Filopodia are slender cytoplasmic projections that extend beyond the leading edge of lamellipodia in migrating cells. Within the lamellipodium, actin ribs are known as microspikes, and when they extend beyond the lamellipodia, they're known as filopodia. They contain microfilaments cross-linked into bundles by actin-bundling proteins, such as fascin and fimbrin. Filopodia form focal adhesions with the substratum, linking them to the cell surface. Many types of migrating cells display filopodia, which are thought to be involved in both sensation of chemotropic cues, and resulting changes in directed locomotion.
The Rho family of GTPases is a family of small signaling G proteins, and is a subfamily of the Ras superfamily. The members of the Rho GTPase family have been shown to regulate many aspects of intracellular actin dynamics, and are found in all eukaryotic kingdoms, including yeasts and some plants. Three members of the family have been studied in detail: Cdc42, Rac1, and RhoA. All G proteins are "molecular switches", and Rho proteins play a role in organelle development, cytoskeletal dynamics, cell movement, and other common cellular functions.
Cell division control protein 42 homolog is a protein that in humans is encoded by the Cdc42 gene. Cdc42 is involved in regulation of the cell cycle. It was originally identified in S. cerevisiae (yeast) as a mediator of cell division, and is now known to influence a variety of signaling events and cellular processes in a variety of organisms from yeast to mammals.
Formins (formin homology proteins) are a group of proteins that are involved in the polymerization of actin and associate with the fast-growing end (barbed end) of actin filaments. Most formins are Rho-GTPase effector proteins. Formins regulate the actin and microtubule cytoskeleton and are involved in various cellular functions such as cell polarity, cytokinesis, cell migration and SRF transcriptional activity. Formins are multidomain proteins that interact with diverse signalling molecules and cytoskeletal proteins, although some formins have been assigned functions within the nucleus.
Transforming protein RhoA, also known as Ras homolog family member A (RhoA), is a small GTPase protein in the Rho family of GTPases that in humans is encoded by the RHOA gene. While the effects of RhoA activity are not all well known, it is primarily associated with cytoskeleton regulation, mostly actin stress fibers formation and actomyosin contractility. It acts upon several effectors. Among them, ROCK1 and DIAPH1 are the best described. RhoA, and the other Rho GTPases, are part of a larger family of related proteins known as the Ras superfamily, a family of proteins involved in the regulation and timing of cell division. RhoA is one of the oldest Rho GTPases, with homologues present in the genomes since 1.5 billion years. As a consequence, RhoA is somehow involved in many cellular processes which emerged throughout evolution. RhoA specifically is regarded as a prominent regulatory factor in other functions such as the regulation of cytoskeletal dynamics, transcription, cell cycle progression and cell transformation.
Ras GTPase-activating-like protein IQGAP1 (IQGAP1) also known as p195 is a ubiquitously expressed protein that in humans is encoded by the IQGAP1 gene. IQGAP1 is a scaffold protein involved in regulating various cellular processes ranging from organization of the actin cytoskeleton, transcription, and cellular adhesion to regulating the cell cycle.
Ras-related protein Ral-A (RalA) is a protein that in humans is encoded by the RALA gene on chromosome 7. This protein is one of two paralogs of the Ral protein, the other being RalB, and part of the Ras GTPase family. RalA functions as a molecular switch to activate a number of biological processes, majorly cell division and transport, via signaling pathways. Its biological role thus implicates it in many cancers.
Deleted in Liver Cancer 1 also known as DLC1 and StAR-related lipid transfer protein 12 (STARD12) is a protein which in humans is encoded by the DLC1 gene.
Rnd1 is a small signaling G protein, and is a member of the Rnd subgroup of the Rho family of GTPases. It is encoded by the gene RND1.
Rho GTPase-activating protein 5 is an enzyme that in humans is encoded by the ARHGAP5 gene.
Rnd3 is a small signaling G protein, and is a member of the Rnd subgroup of the Rho family of GTPases. It is encoded by the gene RND3.
RhoD is a small signaling G protein, and is a member of the Rac subfamily of the family Rho family of GTPases. It is encoded by the gene RHOD.
Rnd2 is a small signaling G protein, and is a member of the Rnd subgroup of the Rho family of GTPases. It is encoded by the gene RND2.
Stress fibers are contractile actin bundles found in non-muscle cells. They are composed of actin (microfilaments) and non-muscle myosin II (NMMII), and also contain various crosslinking proteins, such as α-actinin, to form a highly regulated actomyosin structure within non-muscle cells. Stress fibers have been shown to play an important role in cellular contractility, providing force for a number of functions such as cell adhesion, migration and morphogenesis.
Rac is a subfamily of the Rho family of GTPases, small signaling G proteins. Just as other G proteins, Rac acts as a molecular switch, remaining inactive while bound to GDP and activated once GEFs remove GDP, permitting GTP to bind. When bound to GTP, Rac is activated. In its activated state, Rac participates in the regulation of cell movement, through its involvement in structural changes to the actin Cytoskeleton. By changing the cytoskeletal dynamics within the cell, Rac-GTPases are able to facilitate the recruitment of neutrophils to the infected tissues, and to regulate degranulation of azurophil and integrin-dependent phagocytosis.
Guanine nucleotide-binding protein subunit alpha-12 is a protein that in humans is encoded by the GNA12 gene.
Rho-associated protein kinase (ROCK) is a kinase belonging to the AGC family of serine-threonine specific protein kinases. It is involved mainly in regulating the shape and movement of cells by acting on the cytoskeleton.
mDia1 is a member of the protein family called the formins and is a Rho effector. It is the mouse version of the diaphanous homolog 1 of Drosophila. mDia1 localizes to cells' mitotic spindle and midbody, plays a role in stress fiber and filopodia formation, phagocytosis, activation of serum response factor, formation of adherens junctions, and it can act as a transcription factor. mDia1 accelerates actin nucleation and elongation by interacting with barbed ends of actin filaments. The gene encoding mDia1 is located on Chromosome 18 of Mus musculus and named Diap1.
Alan Hall FRS was a British cell biologist and a biology professor at the Sloan-Kettering Institute, where he was chair of the Cell Biology program. Hall was elected a Fellow of the Royal Society in 1999.
References
- ↑ Ridley AJ (October 2006). "Rho GTPases and actin dynamics in membrane protrusions and vesicle trafficking". Trends in Cell Biology. 16 (10): 522–9. doi:10.1016/j.tcb.2006.08.006. PMID 16949823.
- ↑ Fan L, Yan H, Pellegrin S, Mellor H (March 2015). "The Rif GTPase regulates cytoskeletal signaling from plexinA4 to promote neurite retraction". Neuroscience Letters. 590: 178–83. doi:10.1016/j.neulet.2015.02.010. PMID 25668492. S2CID 23364498.
- 1 2 Fan L, Pellegrin S, Scott A, Mellor H (April 2010). "The small GTPase Rif is an alternative trigger for the formation of actin stress fibers in epithelial cells". Journal of Cell Science. 123 (Pt 8): 1247–52. doi:10.1242/jcs.061754. PMC 2848113 . PMID 20233848.
- ↑ Fan L, Mellor H (February 2012). "The small Rho GTPase Rif and actin cytoskeletal remodelling". Biochemical Society Transactions. 40 (1): 268–72. doi:10.1042/BST20110625. PMID 22260703.
- ↑ Pellegrin, Stéphanie; Mellor, Harry (2005-01-26). "The Rho Family GTPase Rif Induces Filopodia through mDia2". Current Biology. 15 (2): 129–133. doi: 10.1016/j.cub.2005.01.011 . ISSN 0960-9822. PMID 15668168. S2CID 16226672.
- ↑ Goggs R, Savage JS, Mellor H, Poole AW (2013-01-24). "The small GTPase Rif is dispensable for platelet filopodia generation in mice". PLOS ONE. 8 (1): e54663. Bibcode:2013PLoSO...854663G. doi: 10.1371/journal.pone.0054663 . PMC 3554654 . PMID 23359340.
- ↑ Ellis, Sara; Mellor, Harry (November 2000). "The novel Rho-family GTPase Rif regulates coordinated actin-based membrane rearrangements". Current Biology. 10 (21): 1387–1390. doi: 10.1016/s0960-9822(00)00777-6 . ISSN 0960-9822. PMID 11084341. S2CID 18415070.
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