Misplaced Pages

#475524

39-523: Crescentin was discovered in 2009 by Christine Jacobs-Wagner in Caulobacter crescentus (now vibrioides ), an aquatic bacterium which uses its crescent -shaped cells for enhanced motility. The crescentin protein is located on the concave face of these cells and appears to be necessary for their shape, as mutants lacking the protein form rod-shaped cells. To influence the shape of the Caulobacter cells,

78-714: A badminton Olympian, but was undecided about a career through high school. Christine Jacobs-Wagner received her BS degree in biochemistry from the University of Liege . She also received her MS in 1991 and her PhD in 1996 from the University of Liege in Belgium in the field of biochemistry. She then went to work with Lucy Shapiro at Stanford Medical School on a fellowship from the European Molecular Biology Organization . where she studied Caulobacter ,

117-480: A bacterium with a flagellum on one end and a stalk on the other end, beginning her fascination with how bacterial cells can become asymmetrical . From 2004 to 2019, she taught and conducted research as a professor at Yale University. As of 2018, Jacobs-Wagner holds an endowed chair in Yale Medical School and is director of their Microbial Institute. Christine Jacobs-Wagner's major breakthrough has been

156-726: A continuous rod. Eukaryotic intermediate filament proteins assemble into filaments of 8–15 nm within the cell without the need for energy input, that is, no need for ATP or GTP . Ausmees et al. continued their crescentin research by testing whether the protein could assemble into filaments in this manner in vitro . They found that crescentin proteins were indeed able to form filaments about 10 nm wide, and that some of these filaments organized laterally into bundles, just as eukaryotic intermediate filaments do. The similarity of crescentin protein to intermediate filament proteins suggests an evolutionary linkage between these two cytoskeletal proteins. Like eukaryotic intermediate filaments,

195-459: A dimeric coil. Two coils come together side-by-side into a strand. Each strand is paired at its head and tail by another strand, so that it continues like a chain. Two chains of strands pair together side-by-side into a filament. Like eukaryotic intermediate filamenets, the CreS filament is octameric and lacks overall polarity. However, CreS does not show a linker domain in the middle but instead presents as

234-510: A great deal of substructure, including analogs of microfilaments , and that proteins are directed by regulatory processes to locate to specific places within the bacterial cell. She was elected to the National Academy of Sciences in 2015 and has received a number of scientific awards. Christine Jacobs-Wagner grew up in Belgium in a town near Liege . She thought of becoming a cyclist or

273-451: Is delivered ("loaded") to the unclamped end of the other sub-filament, whereupon ATP within the already clamped terminal subunit of the other subfragment is hydrolyzed ("fired"), providing the energy needed to release that arm of the end-tracker, which then can bind another Profilin-ATP-actin to begin a new monomer-addition round. The following steps describe one force-generating cycle of an actoclampin molecular motor: When operating with

312-438: Is derived from acto - to indicate the involvement of an actin filament, as in actomyosin, and clamp to indicate a clasping device used for strengthening flexible/moving objects and for securely fastening two or more components, followed by the suffix - in to indicate its protein origin. An actin filament end-tracking protein may thus be termed a clampin. Dickinson and Purich recognized that prompt ATP hydrolysis could explain

351-518: Is directly transferred to elongating filament ends. The pointed-end of each filament is oriented toward the cell's interior. In the case of lamellipodial growth, the Arp2/3 complex generates a branched network, and in filopodia a parallel array of filaments is formed. Myosin motors are intracellular ATP-dependent enzymes that bind to and move along actin filaments. Various classes of myosin motors have very different behaviors, including exerting tension in

390-407: Is highly dynamic. The actin filament network is arranged with the barbed-end of each filament attached to the cell's peripheral membrane by means of clamped-filament elongation motors, the above-mentioned "actoclampins", formed from a filament barbed-end and a clamping protein (formins, VASP, Mena, WASP, and N-WASP). The primary substrate for these elongation motors is profilin-actin-ATP complex which

429-506: Is loaded into the monomer-insertion site of actoclampin motors. Another important component in filament formation is the Arp2/3 complex , which binds to the side of an already existing filament (or "mother filament"), where it nucleates the formation of a new daughter filament at a 70-degree angle relative to the mother filament, effecting a fan-like branched filament network. Specialized unique actin cytoskeletal structures are found adjacent to

SECTION 10

#1733086070476

468-441: Is strictly a monomer-sequestering protein, the behavior of profilin is far more complex. Profilin enhances the ability of monomers to assemble by stimulating the exchange of actin-bound ADP for solution-phase ATP to yield actin-ATP and ADP. Profilin is transferred to the leading edge by virtue of its PIP 2 binding site, and it employs its poly-L-proline binding site to dock onto end-tracking proteins. Once bound, profilin-actin-ATP

507-422: Is that crescentin lacks certain consensus sequence elements at the ends of the rod domain which are conserved in animal lamin and keratin proteins. The protein has been divided into a few subdomains organized similarly to eukaryotic IF proteins. Not every researcher is convinced that it is a homolog of intermediate filaments, suggesting instead that the similarity might have arisen via convergent evolution. (Indeed,

546-523: Is to discover regulation of the times and places for critical components of the DNA replication and cell division processes so that proliferation control can be understood. Microfilament 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

585-451: The actin polymerization nucleus that is then "loaded" onto the end-tracker before processive motility can commence. To generate a new filament, Arp2/3 requires a "mother" filament, monomeric ATP-actin, and an activating domain from Listeria ActA or the VCA region of N-WASP. The Arp2/3 complex binds to the side of the mother filament, forming a Y-shaped branch having a 70-degree angle with respect to

624-472: The cytosol , thereby forming the ATP-actin monomeric units needed for further barbed-end filament elongation. This rapid turnover is important for the cell's movement. End-capping proteins such as CapZ prevent the addition or loss of monomers at the filament end where actin turnover is unfavorable, such as in the muscle apparatus. Actin polymerization together with capping proteins were recently used to control

663-408: The helices of crescentin filaments associate with the cytoplasmic side of the cell membrane on one lateral side of the cell. This induces a curved cell shape in younger cells, which are shorter than the helical pitch of crescentin, but induces a spiral shape in older, longer cells. Like eukaryotic intermediate filaments, crescentin organizes into filaments and is present in a helical structure in

702-420: The 3-dimensional growth of protein filament so as to perform 3D topologies useful in technology and the making of electrical interconnect. Electrical conductivity is obtained by metallisation of the protein 3D structure. As a result of ATP hydrolysis, filaments elongate approximately 10 times faster at their barbed ends than their pointed ends. At steady-state , the polymerization rate at the barbed end matches

741-467: The Cryo-EM structure of CreS does not display the proposed eukaryotic-like interruption in the rod; see next paragraph.) A number of Cryo-EM structures of crescentin were published in late 2023. The researchers used a nanobody that tags onto one specific part of the filament, so that it's easier to tell where each unit of the filament begins and ends. Two chains of the crescentin molecule pair together into

780-412: The actin cytoskeleton as a scaffold, holding them at or near the inner face of the peripheral membrane . This subcellular location allows immediate responsiveness to transmembrane receptor action and the resulting cascade of signal-processing enzymes. Because actin monomers must be recycled to sustain high rates of actin-based motility during chemotaxis , cell signalling is believed to activate cofilin,

819-449: 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

SECTION 20

#1733086070476

858-424: The actin-filament depolymerizing protein which binds to ADP-rich actin subunits nearest the filament's pointed-end and promotes filament fragmentation, with concomitant depolymerization in order to liberate actin monomers. In most animal cells, monomeric actin is bound to profilin and thymosin beta-4 , both of which preferentially bind with one-to-one stoichiometry to ATP-containing monomers. Although thymosin beta-4

897-542: The barbed end, and the ATP is subsequently hydrolyzed . ATP hydrolysis occurs with a half time of about 2 seconds, while the half time for the dissociation of the inorganic phosphate is about 6 minutes. This autocatalyzed event reduces the binding strength between neighboring subunits, and thus generally destabilizes the filament. In vivo actin polymerization is catalyzed by a class of filament end-tracking molecular motors known as actoclampins . Recent evidence suggests that

936-521: The benefit of ATP hydrolysis, AC motors generate per-filament forces of 8–9 pN, which is far greater than the per-filament limit of 1–2 pN for motors operating without ATP hydrolysis. The term actoclampin is generic and applies to all actin filament end-tracking molecular motors, irrespective of whether they are driven actively by an ATP-activated mechanism or passively. Some actoclampins (e.g., those involving Ena/VASP proteins, WASP, and N-WASP) apparently require Arp2/3-mediated filament initiation to form

975-732: The cell and transporting cargo vesicles. One proposed model suggests the existence of actin filament barbed-end-tracking molecular motors termed "actoclampin". The proposed actoclampins generate the propulsive forces needed for actin-based motility of lamellipodia , filopodia , invadipodia, dendritic spines , intracellular vesicles , and motile processes in endocytosis , exocytosis , podosome formation, and phagocytosis . Actoclampin motors also propel such intracellular pathogens as Listeria monocytogenes , Shigella flexneri , Vaccinia and Rickettsia . When assembled under suitable conditions, these end-tracking molecular motors can also propel biomimetic particles. The term actoclampin

1014-569: The cell in movement. Actin was first discovered in rabbit skeletal muscle in the mid 1940 by F.B. Straub. Almost 20 years later, H.E. Huxley demonstrated that actin is essential for muscle constriction. The mechanism in which actin creates long filaments was first described in the mid 1980. Later studies showed that actin has an important role in cell shape, motility, and cytokinesis. Actin filaments are assembled in two general types of structures: bundles and networks. Bundles can be composed of polar filament arrays, in which all barbed ends point to

1053-444: The cell. Crescentin is necessary for both shapes of the Caulobacter prokaryote (vibroid/crescent-shape and helical shape, which it may adopt after a long stationary phase). The crescentin protein has 430 residues; its sequence mostly consists of a pattern of 7 repeated residues which form a coiled-coil structure. The DNA sequence of the protein has sections very similar to the eukaryotic keratin and lamin proteins, mostly involving

1092-486: 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

1131-407: The coiled-coil structure. Ausmees et al. (2003) proved that, like animal intermediate filament proteins, crescentin has a central rod made up of four coiled-coil segments. Both intermediate filament and crescentin proteins have a primary sequence including four α-helical segments along with non-α-helical linker domains. An important difference between crescentin and animal intermediate filament proteins

1170-492: The depolymerization rate at the pointed end, and microfilaments are said to be treadmilling . Treadmilling results in elongation in the barbed end and shortening in the pointed-end, so that the filament in total moves. Since both processes are energetically favorable, this means force is generated, the energy ultimately coming from ATP. Intracellular actin cytoskeletal assembly and disassembly are tightly regulated by cell signaling mechanisms. Many signal transduction systems use

1209-435: The discovery that the tiny cells of bacteria such as Caulobacter, Escherichia coli , and Borrelia are not simply bags of biochemicals but instead program the locations of their protein components via their regulatory systems. She also discovered the protein crescentin , which forms bacterial intermediate filaments , structures once thought to occur only in eukaryotic cells. The current focus of her laboratory's work

Crescentin - Misplaced Pages Continue

1248-566: The filament built from crescentin is elastic. Individual proteins dissociate slowly, making the structure somewhat stiff and slow to remodel. Strain does not induce hardening of the structure, unlike eukaryotic IFs that do. Christine Jacobs-Wagner Christine Jacobs-Wagner is a microbial molecular biologist. She is the Dennis Cunningham Professor of Biology and Microbiology and Immunology at Stanford University . Jacobs-Wagner's research has shown that bacterial cells have

1287-448: The forces achieved during actin-based motility. They proposed a simple mechanoenzymatic sequence known as the Lock, Load & Fire Model, in which an end-tracking protein remains tightly bound ("locked" or clamped) onto the end of one sub-filament of the double-stranded actin filament. After binding to Glycyl-Prolyl-Prolyl-Prolyl-Prolyl-Prolyl-registers on tracker proteins, Profilin-ATP-actin

1326-461: The pattern created by the binding of myosin S1 fragments: they themselves are subunits of the larger myosin II protein complex . The pointed end is commonly referred to as the minus (−) end and the barbed end is referred to as the plus (+) end. In vitro actin polymerization, or nucleation , starts with the self-association of three G-actin monomers to form a trimer . ATP -bound actin then itself binds

1365-470: The plasma membrane. Four remarkable examples include red blood cells , human embryonic kidney cells , neurons , and sperm cells. In red blood cells, a spectrin -actin hexagonal lattice is formed by interconnected short actin filaments. In human embryonic kidney cells, the cortical actin forms a scale-free fractal structure. First found in neuronal axons , actin forms periodic rings that are stabilized by spectrin and adducin – and this ring structure

1404-421: The rate of ATP hydrolysis and the rate of monomer incorporation are strongly coupled. Subsequently, ADP -actin dissociates slowly from the pointed end, a process significantly accelerated by the actin-binding protein, cofilin . ADP bound cofilin severs ADP-rich regions nearest the (−)-ends. Upon release, the free actin monomer slowly dissociates from ADP, which in turn rapidly binds to the free ATP diffusing in

1443-565: The same end of the bundle, or non-polar arrays, where the barbed ends point towards both ends. A class of actin-binding proteins , called cross-linking proteins, dictate the formation of these structures. Cross-linking proteins determine filament orientation and spacing in the bundles and networks. These structures are regulated by many other classes of actin-binding proteins, including motor proteins, branching proteins, severing proteins, polymerization promoters, and capping proteins. Measuring approximately 6 nm in diameter , microfilaments are

1482-477: The thinnest fibers of the cytoskeleton. They are polymers of actin subunits (globular actin, or G-actin), which as part of the fiber are referred to as filamentous actin, or F-actin. Each microfilament is made up of two helical , interlaced strands of subunits. Much like microtubules , actin filaments are polarized. Electron micrographs have provided evidence of their fast-growing barbed-ends and their slow-growing pointed-end. This polarity has been determined by

1521-542: Was then found by He et al 2016 to occur in almost every neuronal type and glial cells , across seemingly every animal taxon including Caenorhabditis elegans , Drosophila , Gallus gallus and Mus musculus . And in mammalian sperm, actin forms a helical structure in the midpiece, i.e., the first segment of the flagellum . In non-muscle cells, actin filaments are formed proximal to membrane surfaces. Their formation and turnover are regulated by many proteins, including: The actin filament network in non-muscle cells

#475524