Institut de Biologie StructuraleGrenoble / France


New light on bacteriophages, bacteria killers

The vast majority of phages, bacterial viruses, possess a capsid containing the genetic material and a tail that functions in host recognition, cell wall perforation and safe viral DNA transfer from the capsid to the host cytoplasm. But how does the tip of the tail and the capsid communicate? Different teams of the Institute de Biologie Structurale have answered this question by determining the structure of the tail tube by combining cryo electron microscopy and crystallography. This study was published in Nature Communication.

Bacteriophage T5 tail tube structure suggests a trigger mechanism for Siphoviridae DNA ejection. Charles-Adrien Arnaud, Grégory Effantin, Corinne Vivès, Sylvain Engilberge, Maria Bacia, Pascale Boulanger, Eric Girard, Guy Schoehn and Cécile Breyton. Nature Communication doi:10.1038/s41467-017-02049-3

Innovative and specific painting of the pneumococcal surface

Peptidoglycan (PG) is the star molecule of the bacterial wall. In Gram-positive bacteria, teichoic acids (TA) are much less famous, although they account for about half the mass of surface molecules. These glycopolymers, anchored on the plasma membrane or on the PG, are decorated in pneumococcus with phosphorylcholines (PCho). This property was exploited to label the TA so that they could be located and tracked during the cell cycle. The labelling is carried out in two simple steps: culture of pneumococci in the presence of Propargyl-choline then coupling of modified PCho linked to TA with a fluorophore by click chemistry. This efficient and specific labelling, carried out on living bacteria, revealed the probable co-location of molecular machines for the synthesis of PG and TA. It also allows detection of the pneumococcus amongst a bacterial mixture, a pre-requisite for medical diagnostic (Figure). This work initiated within the IBS Pneumococcal Group benefited from an excellent partnership with the Département de Pharmacochimie Moléculaire of the UGA and from a significant contribution to molecular analyses from collaborators in Germany.

Specific and spatial labeling of choline-containing teichoic acids in Streptococcus pneumoniae by click chemistry. Di Guilmi AM, Bonnet J, Peiβert S, Durmort C, Gallet B, Vernet T, Gisch N, Wong YS. Chemical Commununications (Camb);53(76):10572-10575

Channelrhodopsin reveals its dark secrets

Ion channels are integral membrane proteins that upon stimulation modulate the flow of ions across the cell or organelle membrane. Channelrhodopsins (ChRs) appeared to be unusual channels. They belong to the large family of microbial rhodopsins, seven-helical transmembrane proteins containing retinal as chromophore. Photon absorption initiates retinal isomerization resulting in a photocycle, with different spectroscopically distinguishable intermediates, thereby controlling the opening and closing of the channel. In 2003, it was demonstrated that light-induced currents by heterologously expressed ChannelRhodopsin2 (ChR2) can be used to change a host`s membrane potential. This accomplishment has made ChR2 the first long thought and further used a key instrument of neuroscience. ChR2 opened a new field - optogenetics. Optogenetics is a biological technique which involves the use of light to control cells in living tissue, typically neurons, that have been genetically modified to express special light-sensitive proteins. In 2010, optogenetics was chosen as the "Method of the Year" across all fields of science and engineering by the interdisciplinary research journal Nature Methods.

Despite the extraordinary importance of this protein, a high-resolution structure and structural mechanisms of a native ChR2 (and other ChRs) have not yet been known. A step forward was the structure of a chimera between ChannelRhodopsin1 and ChannelRhodopsin 2 (C1C2). However, recent electrophysiological and Fourier transform infrared data showed that C1C2 exhibits light-induced responses that are functionally and mechanistically different from ChR2. Given that ChR2 is the most frequently used tool in optogenetics, a high-resolution structure of ChR2 is of high importance. Deciphering the structure of the native channel would shed light on how the light-induced changes at the retinal Schiff base (RSB) are linked to the channel operation.

In this work scientist from the MEMBRANE group and their collaborators expressed ChR2 in LEXSY expression system and used in meso crystallization approach to determine the crystal structure of the wild-type ChR2 and C128T slow mutant at 2.4 and 2.7 Å, respectively. The determined structures of ChR2 and its C128T mutant present the molecular basis for the understanding of ChR functioning. They provide insights into mechanisms of channel opening and closing.

Thereby, this work shed light to the molecular mechanisms of Channelrhodopsin 2 work and opens the possibilities to make engineering of enhanced optogenetic tools more efficient.

Structural insights into ion conduction by channelrhodopsin 2. Volkov O, Kovalev K., Polovinkin V, Borshchevskiy V, Bamann C, Astashkin R, Marin E, Popov A, Balandin T, Willbold D, Büldt G, Bamberg E, Gordeliy V. Science: Vol. 358, Issue 6366, pp. 1000-1001.