Low-resolution characterization of the 3D structure of the Euglena gracilis photoreceptor Laura Barsanti a , Primo Coltelli b , Valtere Evangelista a , Vincenzo Passarelli a , Anna Maria Frassanito a , Nicoletta Vesentini c , Paolo Gualtieri a, * a Istituto di Biofisica, Via Moruzzi 1, 56124 Pisa, Italy b Istituto Scienza Tecnologia Informazione, Via Moruzzi 1, 56124 Pisa, Italy c Istituto Fisiologia Clinica, Via Moruzzi 1, 56124 Pisa, Italy article info Article history: Received 30 July 2008 Available online 21 August 2008 Keywords: Euglena gracilis Photoreceptor Membrane protein Rhodopsin-like protein abstract This paper deals with the first characterization of the structure of the photoreceptive organelle of the uni- cellular alga Euglena gracilis (Euglenophyta). This organelle has a three-dimensional organization consist- ing of up to 50 closely stacked membrane lamellae. Ionically induced unstacking of the photoreceptor lamellae revealed ordered arrays well suited to structural analysis by electron microscopy and image analysis, which ultimately yielded a low-resolution picture of the structure. Each lamella is formed by the photoreceptive membrane protein of the cell assembled within the membrane layer in a hexagonal lattice. The first order diffraction spots in the calculated Fourier transform reveals the presence of 6-fold symmetrized topography (better resolution about 90 Å A 0 ). The 2D and 3D structural data are very similar with those recently published on proteorodopsin, a membrane protein used by marine bacterio-plankton as light-driven proton pump. In our opinion these similarity indicate that a photoreceptive protein belonging to the same superfamily of proteorodopsin could form the Euglena photoreceptor. Ó 2008 Elsevier Inc. All rights reserved. Life is essentially about information, how information is per- ceived, how it is stored, passed on and used by organisms as they live and reproduce. In the world of photosynthetic microorgan- isms, where virtually all life depends on solar energy, light becomes also a source of information, used to orient microorgan- isms spatially and to guide their movements or growth. Responses using light as a sensory stimulus for orientation towards areas that best match their individual irradiation requirements are thus a vir- tually universal behavior among algae. The full exploitation of light information necessitates proper perceiving devices, able to change the small signal represented by the light falling upon them in a larger signal and response of an entirely different physical nature, i.e. these devices, termed pho- toreceptors, must perform perception, transduction, amplification and transmission. The processing of a photic stimulus and its transformation into an oriented movement can be considered the ‘‘vision” phenome- non of motile algae. True vision involves production of a focused image of the external world, and the optical requirements for an eye probably cannot be satisfied by algae, requiring true multicel- lularity with cell specialization and division of labor. Still, algal ‘‘eyes” resemble the prototypic eye postulated by Darwin [1], and have many similarities with the complex vision systems of higher organisms, since they possess a photoreceptive portion, a pig- mented portion and signal transduction chain components in close contact with the effector. The most common type of algal photoreceptor presumably con- sists of extensive two-dimensional patches of photosensitive pro- teins, present in the plasma membrane in close association with the eyespot, an organelle that acts as a screen device. Very often, the photoreceptor cannot be identified by optical microscopy, while the eyespot can be seen easily detected because of its size and color, usually orange–red. Only in few cases, the photoreceptor is detectable by light microscopy as a distinct organelle in a range size of 1–2 lm, as that present in the divisions of Euglenophyta and Heterokontophyta [2]. All the photoreceptors present in invertebrates and vertebrates as distinct organelle posses a well-characterized three-dimen- sional structure consisting of stacked layers of membrane proteins organized in different spatial configuration and different oligo- meric assembly, with different degrees of mobility in the plane of membrane [3,4]. On the other hand, the structure of algal pho- toreceptors as to the disposition of the photoreceptive proteins inside them has not been yet characterized. Moreover, the nature of the photoreceptive proteins is still under debate. This paper deals with the first characterization of the structure of an algal photoreceptor in our case the photoreceptive organelle 0006-291X/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.bbrc.2008.08.045 * Corresponding author. Fax: +39 050 3152760. E-mail address: paolo.gualtieri@pi.ibf.cnr.it (P. Gualtieri). Biochemical and Biophysical Research Communications 375 (2008) 471–476 Contents lists available at ScienceDirect Biochemical and Biophysical Research Communications journal homepage: www.elsevier.com/locate/ybbrc