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Section 3 Actin Myosin.docx

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Biology 2382B
Robert Cumming

ActinAlso called Microfilaments The thinnest of the microtubules You have microtubules that are the thickestActin which is the thinnest and intermediate filaments that are intermediate in size Actin tend to have cortical function found at the cortexthis is just underneath the plasma membrane Microtubules tend to work farther into the cellJust underneath the plasma membrane is where the actin works A lot of their function has to do with plasma membrane function For things like receptor functions and cell movement endocytosis and cell migrationthese all involve actin Looking at a migrating cells you can see distinct pools of actin Actin can form bundles and networks o Microtbuules can kind of be bundled like in axons and nerveso Actin forms much stronger bundles that are quite stable and strongo Unlike microtubules they can form networks as well branched networksBundles of actin and then bundles of actinThe actin helps to support the plasma membranebecause of that it can do many different things Were going to talk a lot of epithelial cellsskin cells The actin gives you that barrier function and strengthMigrating cells give you different actin structures Actin StructureDifferent types of actin4 alpha Beta and Gama Different types of actin but will lok at Gactin globular mainlyGactin is the monomer which directly polymerizes into Factin filamentous microfilaments Looking at the structure of Gactin it is not symmetricalhas 4 different lobeso If you think of it as a 4 leaf clover that has a notch coming out of one part of ito That notch is an ATp binding siteo The NOTCH also gives it Asymmetry This notch means that its different on both sides leading to polarity When these gactin polymerize they oreitn themselves accordingly which forms an overall polar microfilamentThis ATP binding cleft ends up at theend The little monomrs have an ATP binding cleft always poining in the same directionthe minus end The actin forms a double helix that has a minus end and plus endActin PolarityThere are special functions for the minus and plus end If you add s1 myosin to an actin filament it will decorate the actin The s1 myosin will bind ot the actin filament giving it barbed ends little arrow headso These arrow heads always point towards the minus end If you have a nucleating agent youll again have growth at theend Polymerization of actin filaments occurs preferentially at theendAdding actin coating with S1 myosin acts as the nucleating agento This stabilizes the actin allowing it to grow further not the actual nucleus but helps with the process Like with microtubules the idea of critical concentration o If you have an invitro system o If you start adding gactin in the system eventually youll hit a critical concentration and the gactin will then start to polymerize o At that point youll start making a microfilamento All the new monomers that you add will now give you filamentsanything above that critical concentration will give you that fiament so the concentration of actin endsup staying the same after the C c Gactin has to be in the ATP form to polymerizesimilar to how microtubules have to be in the GTP form to polymerize Its faster if you form nucleusa bunch of monomers will get together these smaller units are relatively stable and aggregate to form the nuclei to allow for polymerization Gactin that will form the nuclei which will allow for polymerizationIf you start with nuclei youll get to steady state faster than if you start without nucleibefore you get to elongation you HAVE TO FORM THE NUCLEI If you start off with nuclei youll be able to start forming elongation faster Nucleationjust having that first little bit of polymerized stuff
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