Source:  Cytoskeleton    Tag:  cytoplasmic streaming
Free online life science study material. Made up of actin filaments or microfilaments, intermediate filaments and microtubules.
Intermediate filaments are absent in plants except for lamins because they have rigid cell wall.

7nm in diameter, filamentous and smallest among the three.
Mainly involved in muscle contraction, locomotion and cell migration, cytoplasmic streaming, cell attachment to extra cellular matrix, cell cell attachment for tissue formation etc.

Actin protein is basic unit of microfilament. It is a globular protein with ATP/ADP binding site. It is U shaped protein. The individual subunits are called G- actin and polymer is called F- actin.
The F actin is a polar structure with pointed and barbed ends.
The initial polymerisation is called nucleation. During nucleation three monomers should fit in right manner to form the filament.
Once three monomers are correctly allighned the subsequent monomers can be added through reversible addition.

At the pointed end there is ADP and at barbed end there is ATP present.For polymerisation hydrolysis of ATP is not required.
It only enhances the rate of asembly.
Actin binding proteins:
1. Formin: These are barbed end tracking proteins which nucleate the actin monomers.Also move along growing polymers.
2.Arp 2/3 : Causes branching of actin filament. Branching is important for cell migration and cell movement.
3.Cofilin: This is required for remodeling of existing actin filament. ADP actin  gets bound to cofilin so reassembly is prevented.
Profilin:Causes exchange of ADP for ATP.The subunit can no longer bound to cofilin and reassembly can take place

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