Symbiosis in trypanosomatids is characterized by the presence of a obligatory intracellular bacterium, which has co-evolved with the host protozoa through a mutualist relationship. The origin of symbiont envelope is controversial; for some authors it is derived from the host plasma membrane, while others believe that it has prokaryotic origin. Previously, we showed that phosphatidylcholine (PC), which is unusual in prokaryotes, is present in the endosymbiont envelope, as described to bacteria that are closely associated to eukaryotes. The biosynthesis of PC has two pathways: one predominant in mammalian cells, where free choline is directly converted to PC via the intermediates choline-phosphate and CDP-choline (Kennedy pathway); and other predominant in prokaryotes, where PC is formed by three successive methylations of Phosphatidylethanolamine (PE) (Greenberg pathway). In this study we investigated metabolic pathways involved in PC production in endosymbiont-bearing trypanosomatids. Our recent studies with Crithidia deanei grown in ³²Pi culture medium showed that PC is the main phospholipid produced. Isolated endosymbionts obtained from protozoa cultivated in ³²Pi containing medium, also presented great amounts of PC. Interestingly, isolated endosymbionts incubated in Warren medium containing ³²Pi for 1 h, were able to uptake this tracer and to synthesize phospholipids, especially PE. However, when isolated symbionts were incubated for 3 hs in ³²Pi culture medium, part of ³²P-PE was converted to ³²P-PC. Taken together, these results suggest that the endosymbiont in trypanosomatids has authonomy to synthesize part of its PC, probably through the Greenberg pathway. New studies are necessary to better characterize the phospholipid pathways involved in this mutualist relationship. In this moment we are also investigating the protein composition of the endosymbiont envelope in order to identify the outer membrane origin.
This work was supported by: CNPq and FAPERJ.