Question

Explain why actin filaments add monomers at the plus end and lose monomers at the minus end AND what function this plays in locomotion.

Answer 1

The major cytoskeletal protein is actin. Individual actin molecules also called G-actin, are globular proteins of 375 amino acids. Structure of G actin is described to have a pointed end and a barbed end. G actin monomers polymerise to form actin filaments. The assembly becomes with the nucleation of 3 monomers to form a trimer. Polymerization is reversible and proceeds from both ends, yielding a filament called F-actin (actin filament). Polymerization is a reversible process in which, monomers both associates with and dissociates from both end of the filament. And high concentration of free subunits, the filament grows at each end. But growth is faster at the barbed end (+ve end). When the concentration of free actin monomers falls below the certain level, the barbed end wil continue to increase in length. But the pointed end (-ve end) will decrease in length. Resulting in a phenomenon called “Treadmilling". Treadmilling in which there is a net gain in subunits at barbed end and an equivalent net loss of subunits at pointed end. Because this cycle is depending on ATP, which is bound to actin monomers added to the end of the filament. Actin monomers at the barbed end are there for bound to ATP. Immediately after polymerization, ATP is hydrolysed to ADP resulting in a zone of ADP bonded monomers towards the pointed end of the F actin. And ADP bonded actin at this -ve end dissociates mor freely than that of ATP bonded actin monomers. In contrast actin bind to ATP associates more readily to rapidly growing barbed(+ve) end.

In the cells, two types of protein including one called "Formin Dimer" determinine where filaments are formed. Formin binds to monomers and facilitate the nucleation process of monomers to form a trimer. After nucleation polymerization proceeds quickly. At the leading edge of moving cells, actin filament treadmills and branch extensively. The branches are nucleated by the "Arp2/3 complex", which binds near the barbed end of filaments and formed a new branch. Arp2/3 promote the remodelling of actin cytoskeleton that required for the cell movements and changes in cell shape.