Archeabacterial phototaxis

Martin Engelhard, Georg Schmies and Ansgar A. Wegener

Table of Contents

Abstract 2
1.1 Introduction 2
1.2 Two-component systems in Archea 6
1.2.1 Chemotaxis in H. salinarum 6
1.2.2 Phototaxis of Halobacteria 8
1.2.3 Receptor/transducer complexes 9
1.3 Properties of sensory rhodopsins and the receptor/transducer complex 10
1.3.1 Primary sequences 11
1.3.2 Absorption spectra 12
1.3.3 Photocycle of sensory rhodopsins 14
1.3.4 Proton transfer reactions of sensory rhodopsins 16
1.3.4.1 Receptors as proton pumps 16
1.3.4.2 Proton transfer reactions in the receptor/transducer complexes 19
1.3.5 Properties of the SR/Htr complex 20
1.4 Molecular mechanism of the signal transfer 22
1.4.1 The receptor-transducer interaction 22
1.4.2 Molecular mechanism of the signal transfer 24
1.5 Outlook 26
Acknowledgements 26
References 26

Abstract

Phototaxis in Archaea is regulated by the two receptors sensory rhodopsin I and sensory rhodopsin II which are closely related to the two ion pumps halorhodopsin (HR) and bacteriorhodopsin (BR). These seven helix membrane proteins are activated by light which induces an all-trans to 13-cis isomerisation of the retinal chromophore bound via a protonated Schiff base to helix G. The signal invoked by these reactions triggers structural changes in cognate halobacterial transducers of rhodopsin. The cytoplasmic domains of these membrane proteins are homologous to that of eubacterial chemoreceptors which activate proteins of the two-component signalling cascade. The similarities between the phototaxis machinery with the two-component signalling chain on the one hand and between the photoreceptors with the ion pumps BR and HR on the other direct the present review. The first part addresses the physiological response of the H. salinarum towards light and the underlying protein network. The next section focuses on the shared properties of receptors and ion pumps such as structural similarities and common principles of the light activated reactions. Finally, the molecular mechanism of signal transfer from the photoreceptor to the transducer is discussed.

1.1 Introduction

Bacteria and Archaea have survived the most dramatic environmental changes that have occurred since their first appearance, three billion years ago. They have occupied almost every ecological niche available, including extremes such as high temperatures at acidic or alkaline conditions. One reason for their endurance is their ability to respond adequately and precisely to environmental changes either genetically or by a locomotive answer. The information flow from the external input across the plasma membrane to the activation of the physiological signal is based on the so-called two-component signalling system that has been found in all three domains of life (for recent reviews on eukaryotic and prokaryotic two-component system see, e.g. This signalling pathway consists of sensors, which receive and transmit the external stimuli to cytoplasmic proteins, including both a histidine and an aspartate kinase (hence the name) which function as transmitter and receiver, the latter regulating the physiological response on the level of genes, proteins, or the cellular motor. The input signal can be quite diverse, ranging from magnetic fields, gravity, or osmolarity to chemicals, starvation, or photons, to name a few.

The two-component signalling cascade has been thoroughly investigated for the chemosensory system of Escherichia coli, Salmonella typhimurium, and related enteric bacteria. In recent years a similar signalling cascade from the archaeal Halobacterium salinarum has been discovered while analysing the mechanism of phototaxis. These archaea have been of particular interest since the discovery of bacteriorhodopsin (BR), the light-activated proton pump, in the early 1970s. The wealth of available information on the function and structure of BR has been reviewed (e.g.; see also a special issue of Biochem. Biophys. Acta,1460 (2000) with a comprehensive discussion of BR and related pigments). Various three-dimensional structures of the BR ground state and intermediates (reviewed in are now accessible and provide a basis for the understanding of the molecular mechanism of the light-activated proton transfer. Furthermore, this data is important in elucidating signal transduction as exemplified in the sensory rhodopsins.

During these investigations on BR three other retinylidene proteins were discovered. Halorhodopsin (HR), an ion pump like BR (both reviewed e.g. in and, was first described and named by Mukohata and co-workers. In subsequent work, HR has been recognised as an inward directed chloride pump and the amino acid sequence has been determined. Since 2000 the tertiary structure of HR has been available at 1.8 Å resolution. The other two pigments, sensory rhodopsin I (SRI) and sensory rhodopsin II (HsSRII), are responsible for the phototaxis of the bacteria and enable them to seek optimal light conditions for the functioning of the ion pumps HR and BR (SRI) and to avoid photo-oxidative stress (HsSRII) (Figure 1). The earliest report on the phototactic behaviour of H. salinarum was published in 1975 although the involvement of retinal proteins was only recognised in subsequent work. At about the same time it was demonstrated that methylation of membrane proteins is involved in the photosensory and chemosensory behaviour of H. salinarum which suggested that a sensory pathway similar to that in E. coli exists.

Research into halobacterial photosensing made a decisive step forward when Spudich and Spudich isolated HR-deficient mutants. These...