Widely exists inEukaryotic cellMiddle, first insmooth muscle cellFound in because it is betweenMuscle cellThick myofilamentandFilamentous muscleIt is called "middle".Intermediate fiber is the most complex of the three types of skeleton fibers.strongSalt solutionAnd nonionicDetergentTreatment can make the rest of the cellsCytoskeletonIt disappears, and only the intermediate fibers remain. It can be seen that the intermediate fibers are the most tenacious and durable among the three types of cytoskeleton.
The intermediate fibrin is basically assembled into intermediate fiber after synthesis, and there are few free monomers.cytoplasmThe middle wire is usually aroundnucleusStart to assemble and stretch to the cell edge andCell plasma membraneOnCell junctionasDesmosome、Hemidesmosometo be connected.Through cell connection, the intermediate filament connects adjacent cells into one.staynuclear membraneIt is composed of a special type of intermediate filament protein membersNuclear fiber layerIt is distributed in the form of orthogonal network and connected to the nuclear membrane through the lamin B receptor located on the inner nuclear membrane.At a certain timePhysiological conditionsNext, atplant cellSimilar intermediate fiber structure also exists in the.
Intermediate fibers haveTissue specificity, different types of cells contain different IFprotein。tumourAfter cell metastasis, the IF of the source cell is still retained, so the IF antibody can be used to identify the source of the tumor.asmammary cancerandGastrointestinal cancer, containingKeratin, so it can be concluded that it comes fromEpithelial tissue。Most cells contain one kind of intermediate fiber, but a few cells contain more than two kinds, such asSkeletal muscle cellscontainDesminandVimentin。
The intermediate fiber is the third skeleton component of the cell, because the average diameter of this fiber is between microtubule andMicrofilamentTherefore, it is called intermediate fiber.Because its diameter is about 10nm, it is also called 10nm fiber.Microtubules and microfilaments are assembled by globular proteins, while intermediate fibers are assembled by long, rod-shaped proteins.Intermediate fiber is the most complex of the three skeleton fibers.
Main differences
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AndmicrotubuleandMicrofilamentIn contrast, intermediate fibers differ in structure and function in at least three aspects.First of all, the intermediate fiber is a fairly stable structure, that is, the intermediate fiber containingDetergentAnd highSalt solutionWhen cells were extracted, the intermediate fibers remained intact.Second, the volume of the intermediate fiber is different from that of the microtubule and microfilament. The diameter of the microtubule is 24 nm, the microfilament is 7 nm, and the intermediate fiber is 10 nm.They are also different in morphology. Microtubules are hollow assembled by α - β tubulin dimerTubular structure, microfilament is made byActinRight hand assembledSpiral fibre, while intermediate fiberSubunityesAlpha helixTo assemble into a rod like structure.[1]Third, the subunits of IF are differentnucleotideThe subunit of microtubuleGTPorgross domestic productThe subunit of microfilament is bound toATPOr ADP combination.However, many details of intermediate fiber assembly are still unclear.
assemble
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Assembly model of intermediate fiber
Whether the intermediate fiber is composed of one monomer protein or the intermediate fiber is assembled by two or even three different protein monomers, the process of assembling the intermediate fiber is basically similar, and the main process is as follows:
1. First, two parallel and mutually alignedPolypeptide chainWinding to form double strandsSuperhelixDimer（coiled-coil dimer）。This process mainly depends on the combination of two hydrophobic parts of intermediate fibrin monomer.
2. The two dimers are then assembled into a tetramer in a reverse parallel and end to end alignment way, that is, the head of one dimer is connected with the tail of the other dimer.Due to the reverse parallel structure (which is different from the assembly of microfilaments and microtubules) during the assembly of tetramers, the two ends of the intermediate fiber are symmetrical, which determines that the intermediate fiber is non-polar.
3. Each tetramer is extended by connecting head and tail, and further assembled intoProtofilament（protofilament）。
4. The two filaments are parallel and intertwined with each other to form protofiber, or octamer, according to the principle of half molecular length interlacing.This half molecular lengthStaggered arrangementIt may be due to the number of heads of various intermediate fibrin monomersArginineSequence but the middle non spiral region L12 has polyargininebinding siteCaused by.
5. Four raw fibers are wound and coiled to form intermediate fibers.[2]
Assembly process of intermediate fiber
Therefore, the final intermediate fiberCross sectionIt is composed of 32 protein monomer molecules.The assembled intermediate fiber hasPolymorphismThe most common is the intermediate fiber assembled by 8 tetramers or 4 octamers.The stem of intermediate fibrin is assembled as the main part of intermediate fiber, forming the core of intermediate fiber, while the non spiral head and tail protrude out of the core, which is the necessary material basis for intermediate fibrin assembly as intermediate fiber.
Different from microfilaments and microtubules, after synthesis of intermediate fibrin, they are basically assembled as intermediate fibers with few free monomers.The intermediate fibrin in cells is affected bychemical modification, includingAcetylation, phosphorylation, etc.
In vitro assembly of intermediate fibers does not require nucleotides and binding proteins, nor does it depend on temperature and protein concentration.But at lowionic strengthUnder slightly alkaline conditions, most intermediate fibers can undergo obvious depolymerization. Once the ionic strength and pH are restored to close to the physiological level, intermediate fibrin will rapidly self assemble to form intermediate fibers.
In vivo, most of the intermediate fibrins are in the polymerization state and assembled to form intermediate fibrils. There are few free tetramers, and there is no corresponding soluble protein library or balance with itTreadmill behavior。[2]
Intermediate fibronectin and its function
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Intermediate fibreBinding proteinIF associated protein (IFAP) is a kind of protein closely related to the intermediate fiber in structure and function, and it is not a structural component of the intermediate fiber itself. It has very important functions, and has certain cells andTissue specificity。IFAP can be used as an intermediate fiber in cellsSupramolecular structureThe normal function of intermediate fibers requires the participation of intermediate fibronectin.
1. Filaggrin This protein can bindKeratinandVimentinIt can aggregate keratin fibers to form large fibrous aggregates. Because it is only expressed in keratinized epithelium, the expression of this protein is keratinized differentiationSpecificityLogo.
2. Trichohyallin can bind keratin to form compact structure, only in hair follicle and tongueepithelial cellsExpressed in.
3. Plankin/cytolinker type intermediate fibronectin, including desmolykinReticulin(plectin) and BPAG1 (ballous pemphigoid antigen l).Reticulin participates in compositionDesmosomeandHemidesmosome, it can also be used inCytoplasmMedium and intermediate fiber combination.BPAG1 is located insideBridge slab, combine with keratin type intermediate fibers and other intermediate fibers, fix them in desmosomes and hemidesmosomes, and play a role in desmosomes and hemidesmosomesadhesionAnd fixing the intermediate fiber.
4. The main function of IFAP300 is also to combine with keratin intermediate fibers. It plays the same role as BPAGI in desmosomes and hemidesmosomes, that is, the intermediate fibers are anchored on desmosomes.But it is different from reticulin in biochemical and immune characteristics.
5. Some other IFAP protein desmoplakin 1 and 2 participate in the formation of desmosomes;HaemoglobinandANCHORINParticipate in the combination of intermediate fiber and membrane;microtubule-binding protein(MTP2) participates in the transverse bridge between intermediate fibers and microtubules.[2]
function
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Since there is no specific intermediate fiberreversibilityAffected drugs (specific tool drugs), so the understanding and understanding of biological functions of intermediate fibers are not deep.along withmolecular biologyandmolecular geneticsresearch methodOfSwift and violentDevelopment, especially adoptiontransgene、Gene knockoutetc.method studyAfter the function of intermediate fibrin and intermediate fibronectin, we have a further understanding of the function of intermediate fibrin.The functions of intermediate fibers are mainly shown in the following aspects.
（1） The function of intermediate fiber is spatiotemporal specific
The formation and function of intermediate fibers are different in different cell lines and different development stages.For example, the epithelial cells of the body can express a variety of keratins, butembryoIn early stage and adult liver, only one type I and type II keratin is expressed in epithelial cells, while tongue, bladder andSweat glandSix or more keratins can be expressed in 60% of epithelial cells.In the skin, it is more typical. Different layers of epithelial cells can express different keratins;Using this feature, the primary site of tumor can be diagnosed clinically.
The intermediate fiber is not easy to break when subjected to large deformation force, and is more resistant than microtubules and microfilamentsShear force。In vitro experimentConfirm epithelial cellsMuscle cellandGlial cellAfter losing the complete intermediate fiberReticular structureAfter that, it is easy to break when encountering shear force.asHereditary disease——In patients with simple epidermolysis bullosa, the expression of keratin gene is defectiveKeratin, resulting in epidermisBasal cellMesokeratin fiber network is damagedMechanical damageVery sensitive. Slight squeezing can destroy the mutant basal cells and cause blisters on the patient's skin.These results indicate that intermediate fibers play an important role in providing mechanical strength of cells.
（3） Maintain cell and tissue integrity
Intermediate fibers, internal and nuclear surfaces andNuclear matrixDirect contact, can communicate withcell membraneandExtracellular matrixDirect contact with microtubules, microfilaments and othersOrganelleTogether form the fiber support network of the cell, which can maintain and fix the specific cell nucleus and various organelles in the cellspatial location , keep cellsmorphological structure Its integrity is conducive to the completion of its functions.Experiments confirm thatHepatocyteCK8 gene knocks out or transfers into the mutated CK18 gene, which destroys the intermediate fiber network structure in the liver cells, resulting in the cells becoming easy to break, and eventually causing liver degeneration, injury, infection and necrosis, indicating that the intermediate fiber can maintain the integrity of the cells.cell divisionAfter,LamininAvailable onEndoplasmic reticulumAnd other cytoplasmic structuresnuclear membraneTo maintain the integrity of the nucleus.Intermediate fibers also play an important role in maintaining tissue integrity.
Intermediate Fibrin and Single ChainDNAThere is a high affinity between intermediate fibrin anddna replication It is related to transcription.Laminin, with the help of other proteins, can interact withChromatinThe binding point may be nuclear matrix adhesion point, DNAReplication siteandtelomere。
（5） Related to cell differentiation and cell survival
In different types of cells or at different stages of cell differentiation, different types of intermediate fibers can be specifically expressed.Intermediate fiber andHistiocyteAn important example of differentiation is the process of epidermal differentiation.epidermisCell differentiationOccurring in the deepestGerminal layer（Basement layer）With the differentiation of cells, cells gradually move to the surface of the epidermis, and finally formKeratinocytesIt falls off from the epidermis.The germinal cells contain a large number of intermediate fiber bundles composed of prekeratin.With the progress of cell differentiation, various keratins expressed at different differentiation stages can be detected separately. When the cell differentiation reaches the final stage, other proteins in the organelle and cytoplasm disappear, only keratin intermediate fibers still exist, indicating that it is related to cell survival.
Because the distribution of intermediate fibers has strict tissue specificity, the tissue origin and differentiation of tumor cells can be identified by identifying the types of intermediate fibers in cells in clinical practice, and the nature of tumors can be determined.
With the further study of intracellular signal transduction, it has been found that intermediate fibers play a certain role in some intracellular signal transduction processes.[2]
