The involvement of a protein kinase in phototaxis and gravitaxis of Euglena gracilis

Abstract

The unicellular flagellate Euglena gracilis shows positive phototaxis at low-light intensities (<10 W/m(2)) and a negative one at higher irradiances (>10 W/m(2)). Phototaxis is based on blue light-activated adenylyl cyclases, which produce cAMP upon irradiation. In the absence of light the cells swim upward in the water column (negative gravitaxis). The results of sounding rocket campaigns and of a large number of ground experiments led to the following model of signal perception and transduction in gravitaxis of E. gracilis: The body of the cell is heavier than the surrounding medium, sediments and thereby exerts a force onto the lower membrane. Upon deviation from a vertical swimming path mechano-sensitive ion channels are activated. Calcium is gated inwards which leads to an increase in the intracellular calcium concentration and causes a change of the membrane potential. After influx, calcium activates one of several calmodulins found in Euglena, which in turn activates an adenylyl cyclase (different from the one involved in phototaxis) to produce cAMP from ATP. One further element in the sensory transduction chain of both phototaxis and gravitaxis is a specific protein kinase A. We found five different protein kinases A in E. gracilis. The blockage of only one of these (PK.4, accession No. EU935859) by means of RNAi inhibited both phototaxis and gravitaxis, while inhibition of the other four affected neither phototaxis nor gravitaxis. It is assumed that cAMP directly activates this protein kinase A which may in turn phosphorylate a protein involved in the flagellar beating mechanism.

Ground-Based Facilities for Simulation of Microgravity: Organism-Specific Recommendations for Their Use, and Recommended Terminology

“…The protists Paramecium (ciliate) and Euglena (flagellate) show distinct gravity-guided modes of behavior in the form of negative gravitaxis (orientation against the direction of the gravity vector) and gravikinesis (regulation of the swimming speed with respect to the swimming direction) (Machemer et al, 1991;Machemer and Brä ucker, 1992;Lebert and Hä der, 1996;Hä der et al, 2005aHä der et al, , 2005bHä der et al, , 2006Hä der et al, , 2010Hemmersbach and Braun, 2007;Daiker et al, 2011). Studies in real microgravity have demonstrated the loss of these responses in a time frame of 1-2 min (Hemmersbach-Krause et al, 1993a, 1993bHemmersbach et al, 1996aHemmersbach et al, , 1996bHä der et al, 2005a).…”

Evidence for gravitactic behaviour in benthic diatoms

“…An explanation for the cessation of upward movement and subsequent (slower) decrease in surface biomass would be that cells stop responding to gravity (Coelho et al, 2011). These results could also be explained by a 'gravity sign change', as observed for the flagellate Euglena gracilis (Daiker et al, 2011).…”

“…Some microalgae can reorient themselves with respect to gravity by using the whole protoplast as a sedimenting body. In the well-studied case of Euglena gracilis, gravisensing is based on a transduction chain triggered by the higher density of the protoplast in relation to the surrounding medium, the resulting pressure on the lower part of the cell membrane, subsequent activation of mechano-sensitive ion channels and a change in the cytosolic calcium concentration and membrane potential (Häder, 1999;Hemmersbach et al, 1999;Daiker et al, 2011).…”

“…The most common form of testing for gravitactic movements in aquatic microorganisms is based on the detection of vertical displacement of cells through lateral observation of a sealed chamber with other conditions homogeneous (Fenchel & Finlay, 1984;Kam et al, 1999;Ritcher et al, 2003;Daiker et al 2011). In some cases, gravitaxis is further confirmed by comparing cell movements in chambers oriented vertically (along the direction of the gravitational stimulus) and in chambers oriented horizontally (rotated 90º, perpendicularly to the gravitational stimulus; Kam et al, 1999) or rotated 180º (inverting the sample relative to the gravitational stimulus; Fenchel & Finlay, 1984).…”

Motion of Euglena gracilis: Active fluctuations and velocity distribution

“…Although we successfully described the population statistics, on the single cell level the mechanism of the active fluctuations and of the observed "fatigue" remains unclear. This mechanism can be investigated further if one addresses complex and not fully understood impulse-response behaviors involving the photoactivated adenylyl cyclase [39,50] and protein kinase [51].…”