Two major classes of actin-nucleating proteins:

1. Formins nucleate the assembly of unbranched filaments 

2. Arp2/3 complex nucleates the assembly of branched actin networks

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Each sarcomere contains two major types of filaments: 

Thick filaments (myosin II)

Thin filaments (actin and associated proteins)

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Sarcomere

Within each skeletal muscle cell are many myofibrils consisting of a repeating array of a specialized structure

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When the Ca2+ levels drop:

Myosin LC is dephosphorylated by MLC phosphatase, causing muscle relaxation.

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Phosphorylation of MLC 

unfolds the myosin II, active and can assemble into bipolar filaments for contraction

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The CaM*-Ca2+ complex binds and activates MLC kinase, phosphorylates the myosin LC. 

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When the Ca2+ level rises, Ca2+ binds CaM.