Nuevos aspectos moleculares de la transducción de la señal luminosa en la bacteria Myxococcus xanthus

Zusammenfassung

The bacterium Myxococcus xanthus has two mechanisms to control the synthesis of carotenoids, a class of pigments that protect the cells against photo-oxidative stress derived from singlet oxygen (1O2) produced upon illumination: a B12-dependent pathway and a B12-independent one. Transduction of the light signal in the B12-independent pathway depends on the CarF protein and the σ-ECF (extracytoplasmic function)/anti-σ factor pair CarQ/CarR. CarF, which is the protein factor that acts most upstream in this signaling pathway, does not resemble any known blue light photoreceptor. Curiously, it has sequence similarity to some eukaryotic proteins (also present in humans) called TMEM189 or Kua, whose function is unknown. How CarF participates in 1O2 detection and how it transduces this signal to inactivate CarR (maybe through light-dependent proteolysis) and release CarQ remains to be established. The resemblance, albeit low, between CarF and some fatty acid desaturases and hydroxylases suggests that its role in carotenogenesis could be related with some change in the fatty acid composition of the membrane. Moreover, given that CarF interacts with CarR and CarR with CarQ, it is possible that these proteins form a multiprotein signaling complex at the membrane that could be linked to the so-called functional membrane microdomains (FMMs) (analogous to eukaryotic lipid rafts). FMMs have specific proteins, called flotillins, which play a central role in their consistence and stability. It is unknown whether, in addition to the carotenogenic response, M. xanthus has other defense mechanisms against photo-oxidative stress that could provoke change in gene expression patterns, beyond those already known. In this work, it has been shown that CarF localizes to the membrane in M. xanthus and that it is required in the light response because it participates in the synthesis of a special class of phospholipids known as plasmalogens. Specifically, CarF is the desaturase, searched for decades, that generates the vinyl ether bond of plasmalogens, which are essential in the response to light (unlike its ether bond precursors). Also, two proteins (ElbD and MXAN_1676), previously related to ether lipid synthesis, have been further analyzed. It has been shown that these proteins are involved in the light response, participating in steps that precede vinyl ether bond generation. Analyses in this work of the subcellular localization and inactivation mechanism of CarR indicate that CarR is present at the membrane both in the dark and in the light, and that its inactivation does not rely on a light-dependent proteolysis process. The study carried out on the presence of FMMs in M. xanthus and its involvement in the light response has shown that M. xanthus has two possible flotillins, with the characteristic flotillin domains and with a subcellular distribution similar to that of other flotillins, such as those from Bacillus subtilis, displaying a punctate pattern in the membrane. Also, this work has revealed that CarR is localized preferably in the membrane fraction usually enriched in FMMs. On the other hand, the global gene expression analysis (microarrays and RNA-seq) have led to the identification of new genes (CarQ-dependent and independent) involved in the light response in M. xanthus. These genes are part of mechanisms related to defense and regulation against 1O2 generated upon photo-oxidative stress. This suggests that other light sensing and signaling mechanisms exist, probably derived from oxidative stress generated by sustained exposure to light.