How do Endosymbionts Evade the Endocytic Cycle: The Story of Planococcus Citri Mealybugs

Planococcus citri mealybugs are an incredibly unique species due to their nested endosymbiosis, in which Moranella endobia resides within Tremblaya princeps. These endosymbionts work together with their host to provide nutritional support throughout its life cycle and onto future offspring. Though what makes these endosymbionts even more interesting is that when viewed form a cell biological perspective, it becomes evident that they should have been exocytosed out of the host millions of years ago. One of the three membranes that surrounds Tremblaya, particularly the outermost vacuolar membrane, acts as the endosomal compartment around the bacteriome. In a traditional case of the endocytic cycle, the contents within the vacuole would be marked by the GTPase proteins Rab5 and Rab7 respectively until the fate of lysosomal digestion occurred. Though what is unique about the vacuolar membrane that surrounds Tremblaya is two things: first is that that membrane is lost and regained upon maternal transmission and secondly the endosymbionts within the membrane are not reaching their lysosomal fate, rather they are being passed down onto future generations. How these endosymbionts can redirect the endocytic pathway can possibly be explained by one of these four mechanisms: 1. Rabex-5 fails to recruit to the membrane of the early endosome, 2. Interruption of Rab7 activation by inhibiting membrane translocation of Ccz1, 3. Failure of the HOPS complex to bind to the late endosome, 4. Inhibition of translocation of ORP1L to the late endosome. Though the four mechanisms outlined above are very clearly regarding a cell biological process, they are not easily testable in a real-world setting. Thus, to adjust and account for this, the early and late endosomal marker proteins (Rab5 and Rab7) were used as the proteins of interest throughout immunofluorescence and western blot experimentation. These experiments revealed significant difficulties in working with commercially made antibodies but more importantly provided insight as to how is best to go forth with this research. In addition to this, qPCR experimentation and Rab7 epitope analysis did reveal that Rab5 and Rab7 are in fact key players in understanding how these endosymbionts are able to evade the endocytic cycle.