Consider the mechanisms through which actin filaments microtubules are formed ei
ID: 265578 • Letter: C
Question
Consider the mechanisms through which actin filaments microtubules are formed either in cells or in vitro (e.g., using purified protein). Which of the following is not a property that is
shared by these two mechanisms?
(a) Although both actin filaments and microtubules can grow from both their plus- and minus-ends, the growth rate of both structures is faster at their plus ends.
(b) Nucleotide triphosphate hydrolysis promotes the depolymerization of actin filaments and microtubules.
(c) Both actin filaments and microtubules depolymerize at their plus ends.
(d) Free protein subunits that are incorporated into actin filaments and microtubules
bind nucleoside triphosphates.
Explanation / Answer
Actin Filaments (F-actin) grow from the polymerization of G-actin monomers.
Actin is a highly abundant (10-100 micromolar on average), ~42 kDa structural protein found in all eukaryotic cells (except for nematode sperm).
Actin filaments are highly dynamic and their polymerization is usually correlated to their disassembly. Generally, actin filament polymerization occurs over three phases: A nucleation phase, an elongation phase and a steady state phase. During the nucleation phase the formation of a stable ‘actin nucleus’ occurs. This is usually comprised of three actin monomers in complex. In the elongation phase monomers are rapidly added to the filament at the (+ve) or barbed end and this is often facilitated by additional elongation factors such as formin. For this process to occur, the (+) end of the filament must be exposed, and this means removal of capping protein.
In vitro studies
Studies focusing on the accumulation and loss of subunits by microfilaments are carried out in vitro (that is, in the laboratory and not on cellular systems) as the polymerization of the resulting actin gives rise to the same F-actin as produced in vivo. The in vivo process is controlled by a multitude of proteins in order to make it responsive to cellular demands, this makes it difficult to observe its basic conditions.
In vitro production takes place in a sequential manner: first, there is the "activation phase", when the bonding and exchange of divalent cations occurs in specific places on the G-actin, which is bound to ATP. This produces a conformational change, sometimes called G*-actin or F-actin monomer as it is very similar to the units that are located on the filament.This prepares it for the "nucleation phase", in which the G-actin gives rise to small unstable fragments of F-actin that are able to polymerize. Unstable dimers and trimers are initially formed. The "elongation phase" begins when there are a sufficiently large number of these short polymers. In this phase the filament forms and rapidly grows through the reversible addition of new monomers at both extremes. Finally, a "stationary equilibrium" is achieved where the G-actin monomers are exchanged at both ends of the microfilament without any change to its total length. In this last phase the "critical concentration Cc" is defined as the ratio between the assembly constant and the dissociation constant for G-actin, where the dynamic for the addition and elimination of dimers and trimers does not produce a change in the microfilament's length. Under normal “in vitro” conditions Cc is 0.1 ?M, which means that at higher values polymerization occurs and at lower values depolymerization occurs.