Motor proteins are molecular machines that convert the chemical energy derived from the hydrolysis of Adenosine triphosphate (ATP) into mechanical work. This work is used to power various processes in the cell like: transporting organelles, chromosome segregation, and muscle fiber contraction. Eukaryotic cytoskeletal motor proteins are generally categorized into three classes or subfamilies named after their first discovered members: myosin (1), dyneins (2), and kinesins (3). The families are defined by sequence similarity in the motor domain. Motor proteins are composed of identical monomers. Each monomer consists of the head or motor domain that contains the filament binding site and the ATPase site and therefore displays strong motor activity. The next part of monomer is a stalk and a tail. Different proteins from various families have versatile functions in the cells.
1.1. Eg5 Motor Protein
Eg5 is a kinesin-like microtubule plus end directed motor discovered in Xenopus laevis egg extracts. It is a member of BimC subfamily of MT motors and localizes to the mitotic spindle where it is thought to be involved in spindle assembly [1]. Eg5 motor protein plays a crucial role in establishing the bipolar mitotic spindle in the cell division. Like all members of the kinesin family it interacts with microtubules and produces forces and motions when hydrolyzing ATP. In contrast to most other kinesins, it forms antiparallel tetramers that in principle could move two microtubules with respect to each other. Eg5 has mainly been studied in cells and in bulk biochemicals assays. So far no single-molecule experiments have been reported. Such experiments have been very successful in elucidating the molecular mechanism of force generation for neuronal kinesin and for various myosins.
1.2 Processivity
Experiments on motor proteins crucially depend on important property of such a system, called processivity. Motor proteins such as kinesin, which can hydrolyze a large number of ATP molecules before detaching from the microtubule, are called processive [2]. Non-processive motors interact just once with actin filaments and detach. Processivity of Eg5 motor protein is still not well known so one of goals of these experiments was to obtain information about the processivity from gliding assays and single molecule experiments. We can define processivity in three different ways. First, it can be chosen as the average number of chemical cycles before detachment from filament; second, it can be defined as the attachment life time of the motor to the filament, third it can be, chosen to be the meant length spanned by the motor on the filament in the single run [5]. In vitro motility assays under special conditions can give information about processivity.