Each actin molecule appears peanut shaped with 4 subdomains separted into two domain lobes by a deep cleft and connected to one other by a short hinge section of the backbone. this structure forms a atPase fold the site where ATP and Mg are bound. There are four states of actin ATP-G-actin. ADP-G-actin. ATP-F-actin and ADP-F-actin ATP-G-actin and ADP-F-actin predominate in a cell In actin the floor of the cleft acts as a hinge that allows the lobes to flex relative to each other. ATP or ADP -binding alter the comformation of g-actin. in fact. without a bound nucleotide G-actin denatures very quickly In the presence of ATP, The addition of ions- Mg,K,or Nat to a solution of G-actin will induce these actin subunits (G-actin) polymerised in a head-to-tail manner into a flexible filament (actin filament, microfilament, MF or F actin
Each actin molecule appears peanut shaped with 4 subdomains separted into two domain lobes by a deep cleft and connected to one other by a short hinge section of the backbone, this structure forms a ATPase fold, the site where ATP and Mg2+ are bound. There are four states of actin: ATP-G-actin, ADP-G-actin, ATP-F-actin and ADP-F-actin, ATP-G-actin and ADP-F-actin predominate in a cell. In actin, the floor of the cleft acts as a hinge that allows the lobes to flex relative to each other. ATP or ADP -binding alter the comformation of G-actin, in fact, without a bound nucleotide, G-actin denatures very quickly. In the presence of ATP, The addition of ions- Mg2+, K+ , or Na+ to a solution of G-actin will induce these actin subunits (G-actin) polymerised in a head-to-tail manner into a flexible filament (actin filament, microfilament, MF or Factin)
Each monomer is rotated by 166 in the filaments which therefore have appearance of two strands of actin molecules in a double-stranded helix. Since each actin subunit has polarity, and all the subunits of an actin filament are pointed in the same direction, the entire MF also has polarity(called the plus and minus ends). At() end, the ATP-binding cleft of an actin subunit is exposed to the surrounding solution; at opposite end, the cleft contacts the next actin subunit. This polarity of actin filaments is important both in their assembly and in establishing a unique direction of myosin movement relative to actin. The polarity of Mf can be demonstrated by EM in So-called decoration?" experiments by binding of myosin S1 head domains, which appear to apiral around the filament and form a series of arrowhead-like decorations. The polar arrowhead points the(-)end, while the barb refers as(+)
Each monomer is rotated by 166o in the filaments, which therefore have appearance of two strands of actin molecules in a double-stranded helix. Since each actin subunit has polarity, and all the subunits of an actin filament are pointed in the same direction, the entire MF also has polarity (called the plus and minus ends). At (-) end, the ATP-binding cleft of an actin subunit is exposed to the surrounding solution; at opposite end, the cleft contacts the next actin subunit. This polarity of actin filaments is important both in their assembly and in establishing a unique direction of myosin movement relative to actin. The polarity of MF can be demonstrated by EM in so-called “decoration” experiments by binding of myosin S1 head domains, which appear to apiral around the filament and form a series of arrowhead-like decorations. The polar arrowhead points the (-) end, while the barb refers as (+) end
2. Actin-associated proteins and organization of mfs Purified actin is capable of polymerising in vitro to MF, but such filaments lack the capability. In contrast MF in living cells are organised into a variety of patterns, which can be organised into highly ordered arrays, loose ill-defined two-or three- dimensional actin networks (even gelation), or tightly held actin bundles, depending on the type of cell and the activity. The organization and behaviour of Mf inside cells is determined by da remarkable variety of actin-associated or related proteins 100 different proteins) that affect the assembly of MF their physical properties, and their interaction with one another and other organelles
2. Actin-associated proteins and organization of MFs Purified actin is capable of polymerising in vitro to MF, but such filaments lack the capability. In contrast, MF in living cells are organised into a variety of patterns, which can be organised into highly ordered arrays, loose ill-defined two-or three- dimensional actin networks (even gelation), or tightly held actin bundles, depending on the type of cell and the activity. The organization and behaviour of MF inside cells is determined by a remarkable variety of actin-associated or related proteins (~ 100 different proteins) that affect the assembly of MF, their physical properties, and their interaction with one another and other organelles
End-blocking (capping Cross-linking Monomer. sequestering Monomers filaments b 式之 Depolymerizing Membrane-binding Filament-severing
Actin-Binding Proteins Relative molectlar Proteins Mass(kDa) Source Monomer-sequestering proteins Profilin 12-15 Widespread mosins Widespread End-block 1g卩 B-Actinin 35-37 Kidney, skeletal muscle Muscle 28-31 acanthamoeba Network-forming proteins Smooth Actin-binding protein(ABP) 2 Platelets, macrophages Gelatin 23-28 Amoebae Bundling proteins Fimbrin 68 Intestinal epithelium, etc. Villin Intestinal epithelium, oocytes Fascin 57 Sea urchin eggs a-Actinin Muscle Filament-severing protei Gelsolin Widespread Fragmin and severin 42 Amoebae, sea urchin eggs Brevin Blood plasma Filament-depolymerizin 18 proteins esprea AD DE Widespread Membrane-binding proteins Dystrophin 427 Skeletal muscle Vinculin 130 Widespread Ponticulin Dictyostelium Note: Many of the actin-binding proteins can have more than one function depend