Lecture 15 - Cytoskeleton_ actin.pdf

Cytoskeleton: actin 1 Lecture 15 BIOL2020 https://www.mechanobio.info/cytoskeleton-dynamics/ Actin filaments (also known as microfilaments) are helical polymers of the protein actin actin subunits, sometimes called globular or G-actin, is a polypeptide carrying a molecule of ATP or ADP Actin...

Cytoskeleton: actin 1 Lecture 15 BIOL2020 https://www.mechanobio.info/cytoskeleton-dynamics/ Actin filaments (also known as microfilaments) are helical polymers of the protein actin actin subunits, sometimes called globular or G-actin, is a polypeptide carrying a molecule of ATP or ADP Actin is an ATPase ● ● ● The terms actin filament , F‐actin , and microfilament are synonyms G-actin monomer actin filaments are helicoidal The way actin monomers assembles provide polarity to the filament + ● ● ● ● At minus end the ATP-binding pocket is exposed At the plus end the ATP-binding pocket is buried in the filament G actin = globular actin F actin = filamentous actin Growth happens mostly from the + end ● Label actin with myosin heads and then mix it with actin monomers Nucleation Is the Rate-limiting Step in the Formation of Actin Filaments two actin molecules bind relatively weakly to each other, but addition of a third actin monomer makes the complex more stable. The filament then undergoes a net addition of subunits at the plus end while simultaneously losing subunits from the minus end. ● so they are always moving towards the plus end actin treadmilling pushes membranes forward https://www.youtube.com/watch?v=_h0bL-Gsikk&ab_channel=RoyalMicroscopicalSociety Phalloidin is a drug that stabilizes F-actin Phalloidin Death cap mushroom Amanita phalloides Fluorescently conjugated Phalloidin Cells dynamically assemble actin filaments into a variety of structures that have different mechanical properties and functions https://www.youtube.com/watch?v= _h0bL-Gsikk&ab_channel=RoyalMi croscopicalSociety Migrating cells make protrusive structures termed filopodia and lamellipodia actin-binding proteins regulate polymerization and length of filaments Polymerization Length Capping proteins prevent G-actin addition and loss G-actin G-actin CapZ Capping protein binds to the plus ends CP-ɑ CP-𝛃 ● https://doi.org/10.1038/s41467-021-25682-5 CP is an heterodimer https://doi.org/10.1016/S0960-9822(03)00559-1 Cofilin cuts actin filaments into small pieces unstable fragments https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2572264/ stable fragments Actin filaments crosslinking make complex structures ● ● filopodia are made of actin bundles lamellipodium has complex actin networks Alpha-actinin is a crosslinking protein that makes F-actin bundles Actinin ● ● ● It is a dimer crosslinkers must have ≧2 actin binding sites it is rigid Filamin is a flexible actin crosslinker protein ● ● ● crosslinkers must have ≧2 actin binding sites Filamin is a dimer Flexibility allows different angles Actin branching proteins make branches ● ● ● The Arp2/3 complex promotes actin branching Arp2/3 is composed of 7 subunits that are structurally similar to G-actin initiates a new branched filament by binding to the side of ta filament and recruiting actin monomers. F-actin structures usually are linked to the membrane actin cytoskeleton is linked to the membrane by ERM proteins ERM proteins Erzin Radixin Moesin ● binding sites for actin and membrane proteins are hidden but revealed when Erzin is phosphorylated https://www.nature.com/articles/nrm2866 Cells dynamically change the activity and localization of actin-binding proteins to assemble complex actin structures Arp2/3 F-actin These are two examples of migrating cells that send their ARP2/3 proteins to the leading edge Summary Actin binding proteins ● ● ● ● affect growth cut filaments bundle filaments link to the membrane actin filaments ● ● ● ● ● ● G actin = globular actin F actin = filamentous actin polarized helical ATPase grow on plus ends Many complex networks ● ● ● Filopodia lamelopodium Many different actin structures with different functions, properties, looks, etc Actin binding proteins are directed to specific places in the cytoplasm