On a chart (growth rate vs actin monomors), draw growth curves for the plus end

Question

On a chart (growth rate vs actin monomors), draw growth curves for the plus end and minus end of actin filaments, if one end has a critical concentration of 2 µM and the other end has a critical concentration of 4 µM. Label which end is the plus end and which is the minus end. Be sure the slope of each growth curve is consistent with the relative dynamics of each end.

B) If you added actin filaments of a defined length at the following concentrations: 1.5 µM, 3.5 µM and 6.5 µM, would you expect the filaments to grow or shrink at their plus and minus ends?

C) TRUE or FALSE. Treadmilling requires that the addition rate at one end of a filament precisely match the subtraction rate at the other end of a filament. Briefly explain your answer.

Explanation / Answer

Actin filaments are globular proteins with multiple functions. This protein is found in all eukaryotes. There are two types of actin proteins, G-actin and F-actin. G-actin is the globular actin and the F-actin is the filamentous actin. Both these actin s play a crucial role in the cellular functions like mobility and muscle contraction.

The three phases of actin growth, in vitro are as follows:

The attached picture below shows growth of actin:

The polarity of actin filaments is generated by the orientation of actin monomers. The monomers, which orient with their cleft in the direction of the similar end of the filament is called negative (-) end. The monomers, which orient with their cleft in the opposite direction of the filament is called positive (+) end.   

The critical concentration (Cc) of actin filaments is defined as the rate at which the rate of polymerization of filaments is equal to the rate of disassociation. At this this concertation, the filaments and monomers will be in equilibrium. The critical concentration plays an important role in nucleation or polymerization. The rate of polymerization is depends on the concentration of free monomers, which is represented with C × kon. The rate depolymerization is independent of free monomers, which is represented with koff. Therefore, at apparent equilibrium, the rate is represented with koff = Cc × kon.

The term treadmilling is defined as elongation of positive (+) end and shortening in negative end (-). The ATP hydrolysis will drive the process of treadmilling in vivo.

The Thymosin is an important protein, which regulate the polymerization of actin filaments. Thereby contribute several functions in the cells, like differentiation, migration and cell proliferation.

The capping proteins, like CapG regulate the three-dimensional growth of actin filaments. For example, the capZ is an end-capping protein that stops the loss or addition of monomers at the end where unfavorable actin turnover occurs.

From the given data,

-

Ka (p/µM*s)

Kd (p/s)

Cc (µM)

barbed (+) end

10

20

2

pointed (-) end

1

4

4

The formula for calculation of polymerization rate at (+) different actin concertation is as follows:

Actin concertation × 10 p/µM*s

So, at barred end:

Therefore, here polymerization takes place. The chart of growth rate vs actin monomers is as follows:

3)

The term treadmilling is defined as elongation of positive (+) end and shortening in negative end (-). The ATP hydrolysis will drive the process of treadmilling in vivo. Thus, the given statement is true.

-

Ka (p/µM*s)

Kd (p/s)

Cc (µM)

barbed (+) end

10

20

2

pointed (-) end

1

4

4

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