COORDINACION GENERAL JURIDICA a) Misión

2.4.1 What is the contractile ring and its role in the

living cell cycle?

Majority of living organisms like humans cells experience physical division dur- ing their life cycle. This process is called the mitosis. During the mitosis, when all the chromosomes and the genetic code of the cell are equally sepa- rated, a ring made of several laments and motor proteins is formed at the middle of the cell [2932]. The building process of this ring remains a mys- tery. After the organelle is formed, it exerts a tensile force when the laments move. The reduction of the size of the organelle leads to the formation of a cleavage furrow [3336] therefore, the complete physical separation of the cell in two daughter cells (cytokenesis). The four basic steps of the mitosis are pictorially shown in the Fig. 2.3.

ˆ During the prophase, the nucleus of the mother cell condenses and the organelle called mitotic spindle (formed by micro tubule and kinesin motor protein) is formed. This organelle helps the choromosome to move during the mitosis process.

ˆ At the metaphase stage, all the chromosomes get ready for the division. They all align themself at the middle of the cell.

Figure 2.3: The four main steps of the mitosis. The cytokinesis which is the nal stage of the physical division of the cell overlaps with the anaphase and telophase. The cytokinesis may start at either anaphase or telophase, depending on the cell, and it nishes shortly after the telophase process.

ˆ At the anaphase, which is the rst step of the cytokinesis2_{, the chromo-}

somes separate into two identical chromosomes carrying the same genes. Those chromosomes migrate each toward the two poles of the cell. ˆ At the telophase, the cell is already almost completely separated. The

mitotic spindle that was condensed before are decondensed and breaks down into two blocks. An organelle called the actomyosin contractile ring is formed at the middle of the cell creating a cleavage furrow on the cell membrane. The actin myosin network constituent of the ring exerts then enough pressure to complete the division of the cell [31, 3740]. The widely accepted minimal constituent of the contractile ring are the active laments and the myosin II motor proteins [29, 30, 39, 4143]. The laments form a bundle network cross-linked by the myosin II protein. The organelle self organises and generates enough force to constrict the cell until its nal separation. The mechanism behind this contraction behaviour is still to be explored. Nowadays, the contractile behaviour of the ring is known to be the major tenet of the cell division process. The intriguing question is How does the contraction occur and what it the cause of the contraction?.

Suggestions in order to understand the contractile behaviour of the ring have been elaborated by some authors all with dierent and interesting models for the ring. The most commonly used is the minimal model of actomyosin net- work to represent the contractile ring(i.e. a random network of actin lament and myosin II). Literature suggests that all the laments are aligned in anti- parallel manner and that the motion of the myosin II is responsible for the contraction [35, 44]. Others claim that the polymerisation and depolymeri- sation of the chains are responsible for the contractile behaviour of the ring [31,45]. Another idea suggests that the ring does not need the activity of the myosin II in order to produce tensile stress [42] and they argue that the bun- dle actin laments contracts when the motor myosin II reaches the ends of the

chains and stops [35]. To have a more clear idea on what is really happening, one need to know rst what the ring is made of.

2.4.2 The actin-laments

The actin lament generally short named F-actin is a microlament found in the cytoskeletal of the cell. It has a polymeric structure and made of many subunits called globular actin (G-actin) [1]. This chain is about 104_{nm of}

length and its persistence length is about 103_{nm. Each of those subunits has}

a mass of 50 KDa3_{and a size of a few nano-meter. The F-actin is formed by the}

polymerisation of the G-actin. Both G-actin and F-actin are polar therefore, we can distinguish the two ends of the F-actin: the + or barbed end and the −or pointed end. The polymerisation mostly occurs at the + end of the chain and the depolymerisation at the − end [1]. Going from the G-actin to the F-actin, the system need to be fuelled by an energy from a chemical process called hydrolysis. The hydrolysis is the chemical process that transforms the molecule Adenosine Triphosphate(ATP) into the Adenosine Diphosphate (ADP). This chemical reaction is able to release an energy in the order of magnitude roughly equal to ∆µ = 25kT = 8.10−10_{J [1]. Since the reaction is}

reversible, we have both polymerisation and depolymerisation at both ends of the F-actin. AT P + H2O | {z } Stored energy ADP + P_i+ ∆µ | {z } Used energy (2.17)

2.4.3 The molecular motors: Myosin II

The motor protein myosin II is a protein found into all eukariotic cells. It is best known for its contribution of the contractile behaviour in muscle cells. It is responsible for the motion of the F-actin as it uses this lament as path way. Moving toward the + end of the chain, it has the ability to transform the chemical energy into mechanical energy and uses it to move along the chain [1,4648]. Almost like man made machine, this protein uses the energy stored by the chemical reaction(hydrolysis) shown in equation (2.17) to generate mo- tion. This motor is known as non-processive4_{. That is why it is suitable for}

reversible cross-linker description. The activity of the motor proteins is fuelled by the ATP [1, 10, 46].

In addition to actin and myosin protein, the ring comprises other proteins. The other polymers components found in the ring are the intermediate la- ments and the microtubules. This latter serve as tracks way for two class of

3_{1KDa(Kilo Dalton) = 1.66 ∗ 10}−24_{g, is a mass unite used by biochemists. It represents}

the mass of one hydrogen atom.

4_{non-Porcessive means that the motor protein attaches and detaches while moving along}

motor proteins: kinesins and dynein. Each of those motors move by dierent mechanisms. Other types of myosin motor move toward the − end of the lament.

2.4.4 Sliding-laments theory

The Sliding lament theory is an hypothesis rst elaborated by Hugh Huxley and his research team [1,10,37, 47,49]. The theory explains well the contrac- tion mechanism in muscle. It is based on laments sliding passing each other driven by the myosin II motor proteins. The appropriate subunit in which this theory is applicable is called sarcomere5_{. It explains well how the motor pulls}

the laments and make them slide in respect to each other. This theory is also sometimes called the cross-bridge theory. It states that the acting lament and the motor protein myosin II are connected together in such a way that they form a network. The cross bridge cycle 6 _{steps of the motor myosin II are:}

1 Working stroke: The motor head bind to the head of the enzyme causing the myosin to be detached from the lament.

2 Unbinding: The change the conformation: The AP T turn into to ADP + Pi, (hydrolyse) releasing energy to it change the myosin II con-

formation become its hight.

3 Recovery stroke: The phosphate group is released.

4 Binding: The ADP is released and the myosin II goes back to its original position but one step further.