Question

3. G-actin and its polymeric form, F-actin, are two of the most abundant proteins in cells and are involved in a myriad of functions ranging from maintaining the structure of microvilli to powering cell movement.

                  a. The terms critical concentration (Cc) and treadmilling can be used to describe both actin filaments and microtubules. Explain the significance of each and explain why treadmilling is not common, if it exists at all, in cells but can be demonstrated in a test tube type of analysis.

                 

                  b. Thymosin beta4, profilin and formin play key roles in F-actin assembly. How does each function in this regard?

                  c. Listeria monocytogenes is a bacterium that can cause a severe form of food poisoning that can lead to death and it can cross the maternal/placenta barrier thus affecting a fetus. How does this bacterium use F-actin as a means to infect cells?

Answer 1

A.Critical concentration is the concentration of the monomers at which the positive end and the negative end of a polymer of actin or microtubue remain constant and there is no change in the length of the filament.

The process by which there is addition of monomersto one end of the polymeric chain and removal from the other end thereby giving a notion of movement along the filament is called treadmilling. The stage in which the growth of the positive end equals the dissociation from the negative end is termed as steadystate treadmilling.

it is easy to demonstrate treadmilling in a test tube because in this system we can have pure actin or other microtubule molecules and start from the nucleation process, to elongation and dissociation at our own will by altering salt concentrations so it can be monitored easily but in a cellular context, the cells maintain a particular ionic concentration of salts which guide the treadmilling process in a different way and there are much more backgrounds than the pure filaments of the experimental setup.

B. Thymosin B4- it is an actin sequestering protein that bind to G-actin in a 1:1 ratio and thus forbid it from polymerization to form actin filaments.

profilin- it is a protein that binds to G actin and aid in filament formation by binding at the positive end. It also acts as a nucleotide excahnge factor thereby charging the filament proteins to associate properly into an actin filament.

formin- formin proteins help in the elongation of the pre existing filaments by removing the capping proteins that hinder filament growth in the positive end.

C.Listeria monocytogenes contains a polypeptide Act A that can aid as a nucleator of actin cyto skeleton thus forming an F-actin tail for the bacteria. This increases the motility of the bacteria manyfolds inside the cell and thus increases its infectivity to a huge extent.