A pseudopod or pseudopodium is a temporary arm-like projection of a eukaryotic cell membrane that is emerged in the direction of movement. Filled with cytoplasm, pseudopodia primarily consist of actin filaments and may also contain microtubules and intermediate filaments. Pseudopods are used for motility and ingestion. They are often found in amoebas.
The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is composed of similar proteins in the various organisms. It is composed of three main components: microfilaments, intermediate filaments, and microtubules, and these are all capable of rapid growth or disassembly depending on the cell's requirements.
Microfilaments, also called actin filaments, are protein filaments in the cytoplasm of eukaryotic cells that form part of the cytoskeleton. They are primarily composed of polymers of actin, but are modified by and interact with numerous other proteins in the cell. Microfilaments are usually about 7 nm in diameter and made up of two strands of actin. Microfilament functions include cytokinesis, amoeboid movement, cell motility, changes in cell shape, endocytosis and exocytosis, cell contractility, and mechanical stability. Microfilaments are flexible and relatively strong, resisting buckling by multi-piconewton compressive forces and filament fracture by nanonewton tensile forces. In inducing cell motility, one end of the actin filament elongates while the other end contracts, presumably by myosin II molecular motors. Additionally, they function as part of actomyosin-driven contractile molecular motors, wherein the thin filaments serve as tensile platforms for myosin's ATP-dependent pulling action in muscle contraction and pseudopod advancement. Microfilaments have a tough, flexible framework which helps the cell in movement.
The Wiskott–Aldrich Syndrome protein (WASp) is a 502-amino acid protein expressed in cells of the hematopoietic system that in humans is encoded by the WAS gene. In the inactive state, WASp exists in an autoinhibited conformation with sequences near its C-terminus binding to a region near its N-terminus. Its activation is dependent upon CDC42 and PIP2 acting to disrupt this interaction, causing the WASp protein to 'open'. This exposes a domain near the WASp C-terminus that binds to and activates the Arp2/3 complex. Activated Arp2/3 nucleates new F-actin.
Podosomes are conical, actin-rich structures found on the outer surface of the plasma membrane of animal cells. Their size ranges from approximately 0.5 µm to 2.0 µm in diameter. While usually situated on the periphery of the cellular membrane, these unique structures display a polarized pattern of distribution in migrating cells, situating at the front border between the lamellipodium and lamellum. Their primary purpose is connected to cellular motility and invasion; therefore, they serve as both sites of attachment and degradation along the extracellular matrix. Many different specialized cells exhibit these dynamic structures such as invasive cancer cells, osteoclasts, vascular smooth muscle cells, endothelial cells, and certain immune cells like macrophages and dendritic cells.
A growth cone is a large actin-supported extension of a developing or regenerating neurite seeking its synaptic target. It is the growth cone that drives axon growth. Their existence was originally proposed by Spanish histologist Santiago Ramón y Cajal based upon stationary images he observed under the microscope. He first described the growth cone based on fixed cells as "a concentration of protoplasm of conical form, endowed with amoeboid movements". Growth cones are situated on the tips of neurites, either dendrites or axons, of the nerve cell. The sensory, motor, integrative, and adaptive functions of growing axons and dendrites are all contained within this specialized structure.
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.
ADF/cofilin is a family of actin-binding proteins associated with the rapid depolymerization of actin microfilaments that give actin its characteristic dynamic instability. This dynamic instability is central to actin's role in muscle contraction, cell motility and transcription regulation.
Cortactin is a monomeric protein located in the cytoplasm of cells that can be activated by external stimuli to promote polymerization and rearrangement of the actin cytoskeleton, especially the actin cortex around the cellular periphery. It is present in all cell types. When activated, it will recruit Arp2/3 complex proteins to existing actin microfilaments, facilitating and stabilizing nucleation sites for actin branching. Cortactin is important in promoting lamellipodia formation, invadopodia formation, cell migration, and endocytosis.
CapZ, also known as CAPZ, CAZ1 and CAPPA1, is a capping protein that caps the barbed end of actin filaments in muscle cells.
ENA/VASP homology proteins or EVH proteins are a family of closely related proteins involved in cell motility in vertebrate and invertebrate animals. EVH proteins are modular proteins that are involved in actin polymerization, as well as interactions with other proteins. Within the cell, Ena/VASP proteins are found at the leading edge of Lamellipodia and at the tips of filopodia. Ena, the founding member of the family was discovered in a drosophila genetic screen for mutations that act as dominant suppressors of the abl non receptor tyrosine kinase. Invertebrate animals have one Ena homologue, whereas mammals have three, named Mena, VASP, and Evl.
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.
FYVE, RhoGEF and PH domain-containing protein 1 (FGD1) also known as faciogenital dysplasia 1 protein (FGDY), zinc finger FYVE domain-containing protein 3 (ZFYVE3), or Rho/Rac guanine nucleotide exchange factor FGD1 is a protein that in humans is encoded by the FGD1 gene that lies on the X chromosome. Orthologs of the FGD1 gene are found in dog, cow, mouse, rat, and zebrafish, and also budding yeast and C. elegans. It is a member of the FYVE, RhoGEF and PH domain containing family.
Actin-related protein 3 is a protein that in humans is encoded by the ACTR3 gene.
Vasodilator-stimulated phosphoprotein is a protein that in humans is encoded by the VASP gene.
Neural Wiskott–Aldrich syndrome protein is a protein that in humans is encoded by the WASL gene.
The Actin assembly-inducing protein (ActA) is a protein encoded and used by Listeria monocytogenes to propel itself through a mammalian host cell. ActA is a bacterial surface protein comprising a membrane-spanning region. In a mammalian cell the bacterial ActA interacts with the Arp2/3 complex and actin monomers to induce actin polymerization on the bacterial surface generating an actin comet tail. The gene encoding ActA is named actA or prtB.
Actin remodeling is the biochemical process that allows for the dynamic alterations of cellular organization. The remodeling of actin filaments occurs in a cyclic pattern on cell surfaces and exists as a fundamental aspect to cellular life. During the remodeling process, actin monomers polymerize in response to signaling cascades that stem from environmental cues. The cell's signaling pathways cause actin to affect intracellular organization of the cytoskeleton and often consequently, the cell membrane. Again triggered by environmental conditions, actin filaments break back down into monomers and the cycle is completed. Actin-binding proteins (ABPs) aid in the transformation of actin filaments throughout the actin remodeling process. These proteins account for the diverse structure and changes in shape of Eukaryotic cells. Despite its complexity, actin remodeling may result in complete cytoskeletal reorganization in under a minute.
Paracytophagy is the cellular process whereby a cell engulfs a protrusion which extends from a neighboring cell. This protrusion may contain material which is actively transferred between the cells. The process of paracytophagy was first described as a crucial step during cell-to-cell spread of the intracellular bacterial pathogen Listeria monocytogenes, and is also commonly observed in Shigella flexneri. Paracytophagy allows these intracellular pathogens to spread directly from cell to cell, thus escaping immune detection and destruction. Studies of this process have contributed significantly to our understanding of the role of the actin cytoskeleton in eukaryotic cells.
WH1 domain is an evolutionary conserved protein domain found on WASP proteins, which are often involved in actin polymerization.
