Actin-binding proteins can modify the properties of actin. You purify an actin-b

Question

Actin-binding proteins can modify the properties of actin. You purify an actin-binding protein called Abp. You examine the effect of Abp on actin polymerization by measuring the kinetics of in vitro actin filament formation, in which pure actin is added in the presence and the absence of purified Abp protein. You obtain the results shown in the figure below.

Further experiments show that Abp binds to the side of actin filaments and preferentially binds to ADP-containing actin filaments. Propose a possible molecular mechanism consistent with the data above.

Explanation / Answer

In any cell, the elongation or the polymerisation of actin filaments is regulated by a special class of proteins known as actin binding protein. These actin binding proteins control the rate of polymerisation of actin filaments, their hydrolysis and finally their depolymerisation (all these three process together known as treadmilling of actin filaments).

In general, growing actin filament has two sides, one fast growing side known as plus (+) end and other is rather slow growing side called minus (-) end. This asymmetry arise due to differential ATPase activity of actin. During polymerization the G actin is activated by binding to ATP. The + end has ATP-G actin which polymerises to F actin utilizing the energy of ATP, and the result is ADP-F actin at the – end.

As the question clearly states that the Abp bind basically to ADP and as explained above the Abp is proposed to bind at slow growing end of actin filament i.e. – end.

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