Microfilaments are composed ofActin(Actin) molecule spiral polymerized filaments with a diameter of about 7nm, also known as actin filaments, andmicrotubuleandIntermediate fibreJoint compositionCytoskeleton, is a kind of allEukaryotic cellWith a molecular weight of about 42kDaprotein, is also a highly conservative protein, due to species differences (e.galgaeThe difference with human will not exceed 20%.Microfilaments attach, spread, moveEndocytosis、cell divisionAnd many other cell functions play an important role.[1]
The main functions of the microfilaments are: the microfilaments are gathered into bundles along theCytoplasmic circulationThey are arranged in the same direction and control the cytoplasmic circulation of cells.pollen tubeThe growth of actin is also related to microfilaments.[2]
Microfilament is a fiber composed of double stranded actin filaments in the form of spiral, with a diameter of 7nanometer，pitch36 nm, two strands of actin filament are in the same direction.Actin fiber is also a kind ofPolar molecule, has two different ends, one isPositive end, the other isNegative terminal。
Microfilament and itsBinding protein(binding protein) andMyosin(myosin) chemistry of the threemechanical system, usingchemical energyproducemechanical movement。The microfilament bundles formed by microfilaments are calledStress fibre, often across the long axis of the cell.vertebrateActin is divided into three types: α, β and γ, and α type is distributed in myocardium andStriated muscleIn cells, α and γ are distributed insmooth muscle cellIn, β and γ are distributed in nonmuscle cellsMedium.Polymeric and non polymericActinIt can interact with a variety of binding proteins, which can regulate the polymerization of actin and the stability, length and distribution of microfilaments.[3]
Actin monomer (also called G-Actin, full nameGlobular actinGlobular Actin (hereinafter referred to as G actin) is spherical, and there is aATPbinding site。The actin monomer is connected into a string of actin chains one by one, and two such actin chains are intertwined and twisted into a strand of microfilament.This actinPOLYMERIt is also called F-Actin (Fibrous Actin).
Microfilaments were first discovered inmuscle cellsIn the striated muscle andCardiac myocyteMiddle actin bundlesMyofibril, with contraction function.Microfilaments also exist widely in non muscle cells.staycell cycleTheir morphology and distribution can be changed at different stages or when cells flow.Therefore, the microfilament sibling of non muscle cellsmicrotubuleSimilarly, in most cases, it is a dynamic structure that adapts to the needs of cell activities in different structural forms.[3]
Assembly and disassembly
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Microwires can be assembled and disassembled.When the monomer is combined withATPThe monomer tends to polymerize intoPOLYMERIs assembly.And whenATP hydrolysisbecomeADPAfter that, the monomer affinity will decrease, and the polymer tends to depolymerize, that is, to reassemble.High ATP concentration is conducive to the assembly of microfilaments.So whencytoplasmWhen the solution rich in ATP is put into it, the cytoplasm will quickly solidify into glue due to the mass assembly of microfilaments.The assembly speed of both ends of the microfilament is different.The fast end (+pole) is 5 to 10 times faster than the slow end (- pole).When ATP concentration reaches a certain levelcritical valueIt can be observed that the+pole is assembled while the - pole is assembled at the same time“Treadmill behavior”。[3]
process
Microfilament[4]
The assembly of microfilaments can be divided into three stages: nucleation phaseGrowth period(growth phase) or extended period, and balance period(equilibrium)。Nucleation period is the speed limiting process of microfilament assembly, which requires a certain time, so it is also calledDelay period, at this timeActinWhen polymerization starts, its dimer is unstable, easy to hydrolyze, and only formsTrimerOnly stable, that is, core formation.Once the core is formed, globular actin will quickly polymerize at both ends of the core and enter the growth period.The assembly speed of the two ends of the microfilament is different, and the assembly speed of the positive end is significantly faster than that of theNegative terminal, about 10 times more than the negative end.The microfilament extends to a certain period,ActinThe speed of incorporation of the microfilament and its dissociation from the negative end of the microfilament reach a balance. At this time, the length of the microfilament is basically unchanged, and the length of the positive end is equal to the length of the negative end shortened, and the polymerization and dissociation activities are still going on.
The assembly of microfilament can be carried out by steppingCar model(dealing model) and dynamic instability, but the latter is more reasonable.ATP is a dynamic instability regulating the assembly of microfilamentsa sexual behaviorThe main factors of.In addition,Actin-binding proteins (actin-binding protein,ABP）It also regulates the assembly of microfilaments.[1]
adjust
The assembly and disassembly of microfilaments are regulated by a variety of proteins in the cytoplasm. These proteins can bind to the microfilaments and affect their assembly and disassembly speed. They are called microfilament binding proteins (associationsprotein）。
Microfilament[5]
The assembly of microfilaments requires "nucleation" first（nucleation）That is, several monomers polymerize first, and other monomers combine with them to form larger polymers.Arp complex (Action related protein) is a kind of energyActinIt acts as a template to promote the polymerization of actin.Arp complex is composed of Arp2, Arp3 and other five proteins.[1]
The end blocking protein is the "cap" at both ends of the microfilament.When thisprotein binding When onto the microfilament, the assembly and disassembly of the microfilament will stop.This is important for some proteins with fixed length, such asFilamentous muscle。
And forwardfibrin(Profilin, or TranslatorGActin binding protein）Is to promote polymerization, and correspondingly, the proteins that promote depolymerization arecofilin（Cofilin）。Filament cutting protein（filamentsevering protein），asSolGelsolin can cut microfilaments from the middle.Vinculin can fix microfilaments tocell membraneUp, formingAdhesive spot。cross-linker(Cross linking protein) There are more than twoActinThe binding site plays the role of connecting the microfilaments,Tow protein(fimbrin) helps the filaments form bundles, whileFilamentous protein(filamin) cross-linked the microfilaments into a network.[1]
Function of microfilament
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Microfilament removal participates in formationMyofibrilIn addition, it has the following functions:
small intestineepithelial cellsThe free surface ofmicrovilli(microvilli), its axis is a bunch of parallel microfilaments, the positive end of the microfilament bundle points to the top of the microvilli, and the lower end ends at the terminal web.Microfilament bundles support the morphology of microvilli.Since the microfilament bundle does not containMyosin, tropomyosin and α - actin, so the microfilament bundle has no contractility.[3]
Cell deformation movement
CellularDeformation movementMicrofilaments play a key role in.The process can be divided into the following four steps:
② An adhesive spot is formed at the position where the foot contacts the matrix.
③ Under the action of myosin, the microfilament fibers slide, making the cell body move forward.
④ Remove the sticky spots behind the cells.In this way, the cells move forward.Amoeba、white blood cell、FibroblastCan move in this way.[1]
Cytokinesis
At the end of mitosis, two are about to separateDaughter cellInternally generatedShrink ring, shrink ring is composed of parallel arranged microfilaments and myosin II
Cytokinesis[1]
。As the contraction ring shrinksCytoplasmSeparation, atCytochalasinUnder the condition of existence, it is impossible to form a cytokinesis ring, so it forms a binuclear cell.[1]
Acrosome reaction
When sperm and egg combine, microfilamentAcrosomeIt protrudes into the colloid of the egg, and after fusionZygoteCellsSurface areaAccretionmicrovilliThe microfilaments participate in the formation of microvilli, which is conducive to the absorption of nutrients.[1]
Other functions
asOrganelleSportsplasma membraneLiquidityCytoplasmic circulationAll of them are related to the activity of microfilaments. The drug (cytochalasin) inhibiting microfilaments can enhance the membrane flow and destroy the cytoplasmic circulation.[4]
The microfilament fiber can be cut off and combined with inhibition at the end of the microfilamentActinAdd to microfilament fiber,SpecificityInhibition of microfilament function.[1]
Cytochalasins are the metabolic products of fungi. When combined with microfilaments, they can cut off the microfilaments and bind at the end of the microfilaments to inhibit the polymerization of actin at this site, and stop the assembly of microfilaments. Therefore, treatment of cells with cytochalasin can destroy the microfilamentsthree-dimensional spacenetwork structureAnd prevent cell movement, but have no obvious effect on disaggregation.[6]
It can specifically combine with the microfilament to stabilize the microfilament fiber and inhibit its function.Fluorescent labelingOfGhost pen cyclic peptideSpecific display microfilaments.[1]
Philloidin is a bicyclic rod peptide extracted from fungi, which has strong affinity with microfilaments. Fluorescent labeled phalloidin can clearly show the microfilaments in cells.Ghost pen cyclic peptide can enhance the stability of actin fiber and inhibit depolymerization, thus preventing microfilament degradation.experimental study It was found that ghost pen cyclic peptide only binds to F-actin, but not to G-actin.[6]
