Research Topics

Guidance mechanisms in bacteria and sperm

Signal transduction in bacterial chemotaxis

We explore signal transduction strategies using chemotaxis of Escherichia coli as a model. Bacterial chemotaxis is a sophisticated system that integrates many different signals into a common output — a change in the direction of flagellar rotation. The signal transduction in E. coli chemotaxis is between two supramolecular complexes: the receptor complex and the flagellar-motor complex. The receptor supramolecular complex includes the receptors and the enzymes that modulate the receptor activities as well as the enzymes that are modulated by the receptors. The flagellar-motor supramolecular complex includes the motor and its gearbox, termed a switch. A small protein, the excitatory response regulator CheY, shuttles back and forth between the two supramolecular complexes and transduces sensory information between them (Figure 1). Our research is focused on CheY and the switch-motor complex.

Figure 1. A simplified scheme of signal transduction in bacterial chemotaxis of E. coli. Black arrows stand for regulated protein-protein interactions. CheY is a response regulator, CheA is a histidine kinase, and CheZ is a phosphatase. The scheme is not drawn to scale.

CheY. The activity of CheY is regulated by phosphorylation. We found that this protein also undergoes lysine-acetylation, achieved by two different mechanisms, and that acetylation mutants are chemotaxis-defective. We address the question why two covalent modifications (phosphorylation and acetylation) are needed for controlling the activity of this small protein, focusing on revealing the molecular mechanism of acetylation in vivo and on the function that this acetylation fulfills in chemotaxis.

Switch-motor complex. The bacterial flagellar motor is one of the most amazing nano-machines known in nature. This is an electrical motor, located at the base of the flagellum and composed of a rotor and a stator. The rotor contains the protein FliF as well as the three switch proteins — FliG, FliM and FliN. The switch proteins are involved in flagellar assembly, torque generation, and switching the direction of flagellar rotation. The stator is built from the proteins MotA and MotB, acting as torque-generating units. Recently it became apparent that a substantial number of proteins, hitherto unconsidered motor-associated, bind to the motor and affect its function. Of special interest are energy-linked proteins (fumarate reductase, the β subunit of FoF1 ATP synthase, and the NuoCD subunit of NADH-ubiquinone oxidoreductase), found by us to be associated with specific switch proteins of the motor, affecting the direction of flagellar rotation and, in some cases, even flagellar assembly. We currently focus on revealing the function of these interactions and on the question whether ‘Power Packs’ are associated with the flagellar motor.

Sperm guidance in mammals

Contrary to a prevalent belief, there appears to be no competition in the mammalian female genital tract between large numbers of sperm cells racing towards the egg. Instead, small numbers of the ejaculated sperm cells enter the Fallopian tube and these few must be guided in order to make the remaining long, obstructed way to the egg. We revealed two active guidance mechanisms: chemotaxis and thermotaxis. Both mechanisms are restricted to capacitated sperm cells, namely to cells that reached a maturation stage at which they can penetrate the egg and fertilize it. We found that both the egg and its surrounding cumulus cells secrete sperm chemoattractants, and that a temperature difference is established at ovulation in the female’s oviduct as a consequence of a temperature drop at its lower part (isthmus). It is thought that, in vivo, thermotaxis is a long-range mechanism, guiding sperm cells in the Fallopian tube towards the fertilization site, and chemotaxis is a short-range mechanism that is mainly functional at close proximity to the egg (Figure 2).

Figure 2. A scheme of the female genital tract demonstrating the location of sperm thermotaxis and chemotaxis.

Sperm chemotaxis. We recently revealed the behavioral mechanism of chemotaxis of human sperm cells. We demonstrated that hyperactivated motility — a vigorous motility type with large amplitudes of head displacement whose function had been unknown at large — is part of the chemotactic response, causing sperm cells to change their swimming direction. We further found that human sperm cells detect the chemical gradient of the chemoattractant over time rather than over space, meaning that they have kind of a primitive memory. Additionally, we proposed a model for the behavioral mechanism of human sperm cells in a spatial chemoattractant gradient. Our current efforts are focused on identification of the chemoattractant secreted from the egg.

Sperm thermotaxis. We recently revealed a few of the molecular components involved in thermotaxis of human sperm cells. We also found that human sperm cells can respond thermotactically within a wide temperature range (at least 29–41°C), that within this range they preferentially accumulate in warmer temperatures rather than at a single specific, preferred temperature, that they can respond to both ascending and descending temperature gradients, and that they can sense and thermotactically respond to temperature gradients as low as <0.014°C/mm. This temperature gradient is astonishingly shallow because it means that as a spermatozoon swims through its entire body length (0.046 mm) it can sense and respond to a temperature difference of <0.0006°C! We are currently studying the molecular mechanism underlying this extraordinary temperature sensitivity.