Molecular Motors

Introduction

Directed movement is crucial for various physiological processes like muscle contraction, cell division or transporting intracelluar cargo to its point of destination1. To perform these many different tasks, cells have evolved a myriad of molecular motor proteins capable of converting chemical energy into mechanical energy.

Important classes of molecular motors are the Actin-associated Myosins and the Microtubule-associated Kinesins. Most members of these families are dimeric molecules with two catalytic motor domains that are joined by an extended coiled-coil stalk. The bigger part of both the Myosin and the Kinesin family is involved in long-range intracellular transport processes, shuttling cellular cargo along a track system of cytoskeletal filaments. A major feature of transport motor proteins is their ability to walk processively, meaning they can travel for up to several micrometer without dissociating from their track. The work of many research groups on the paradigms of molecular motors, Kinesin-1 and Myosin-V, provides us with a detailed picture of how these motors work. Yet, it remains unclear to which extent those findings can be transferred to other members of the Myosin and Kinesin family.


1To assure oneself of the latter, consider the following example: It would take over 140 years for a globular protein of 4 nm diameter to cover a distance of 1 m (which is about the length of the longest axon in the human body) by diffusion!

 

 

Polystyrene beads (blue) coated with kinesin are captured in a focussed laser beam (yellow) and brought into contact with a surface bound microtubule (grey).

Optical Tweezers

Our laboratory uses Single-Molecule Force Spectroscopy to study the mechanical properties of individual motor proteins both of members of the Kinesin and Myosin family. Knowledge on how molecular motors are affected by external load is crucial for understanding the fundamental working principles of these enzymes. Optical tweezers provide an ideal tool to study the mechanics of individual motor molecules. Our setup is equipped with a force feedback system, allowing to apply constant loads while following the movement of single motor molecules with nanometer precision and millisecond temporal resolution.

 

 

Heterodimeric Kinesin

(in Collaboration with Dr. Zeynep Ökten and Prof. Manfred Schliwa, Adolf-Butenand Institut, LMU München)

This record demonstrates sucessive 8 nm steps for heterodimeric kinesin-2 from C.Elegans, providing direct evidence for its processivity.

In our present studies we focus on the working mechanism of the heteromeric kinesin-2 motors which are involved in the building and maintenance of ciliary structures. These kinesins are unique in that they are composed of two different polypeptides. However, the biological reason for heterodimerization has remained elusive.

Our single molecule optical trapping provide evidence that a heterodimeric kinesin-2 from C.Elegans combines its two subunits to suffice two important aspects for motor function: One subunit is responsible for generating a processive motor, the other one functions as regulatory switch. [1, 2]

 

 

Myosin-V

The forward motion of many molecular motors has already been extensively studied. In our experiments we investigated the motion of Myosin-V under both high backward and forward loads. We find a pronounced asymmetry in the response of Myosin-V to high backward and forward forces. High backward loads can induce processive backward stepping independent of ATP hydrolysis. In contrast forward forces can not induce ATP-independent forward steps. [3]

 

1) Melanie Brunnbauer, Felix Mueller-Planitz, Süleyman Kösem, Thi Hieu Ho, Renate Dombi, J Christof M Gebhardt, Matthias Rief, Zeynep Okten, P Natl Acad Sci Usa, 2010 vol. 107 (23) pp. 10460-5

2) J Christof M Gebhardt, Anabel E-M Clemen, Johann Jaud, Matthias Rief, Proc Natl Acad Sci USA, 2006 vol. 103 (23) pp. 8680-5

3) J Christof M Gebhardt, Zeynep Okten, Matthias Rief, Biophys J, 2010 vol. 98 (2) pp. 277-81