Proteins affecting filament growth

In the resting neutrophil, about 50% of the actin is present in filaments within the cytoskeleton (and hence insoluble in detergents such as Triton X-100), whereas the remainder is detergent soluble and hence is not associated with the cytoskeleton. Data from studies of actin polymerisation in vitro predict that almost all of the actin within the cell should be F-actin (i.e. present in microfilaments). Upon stimulation of neutrophils with agonists such as fMet-Leu-Phe or PMA, actin polymerisation is activated extremely rapidly. There are two important questions Firstly, how is actin maintained in the unpolymerised state in resting cells Secondly, how is it rapidly assembled into the cytoskeleton during activation The answers to these questions lie in understanding the functions of the numerous proteins involved in the assembly and disassembly of actin filaments (Table 4.1). 

Myosin binds F-actin and causes contraction 480 Ca2+/calmodulin 

Gelsolin blocks barbed ends of actin filaments 84 Ca2+,PIP2 

Profilin maintains G-actin monomer pool 15 PIP2 

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