Protein trafficking in African trypanosomes: the role of localization of the serum resistance associated protein in the inhibition of trypanosome lytic factor-1

Abstract

Trypanosoma brucei or African trypanosomes are unicellular parasites that cause the veterinary wasting disease Nagana, and the human disease, Human African Trypanosomiasis. The veterinary pathogen, T. b. brucei, is unable to infect humans due to a trypanolytic subclass of high-density lipoprotein found in human serum called Trypanosome Lytic Factor (TLF-1). Conversely, the human infectious subspecies of African trypanosomes have evolved mechanisms of resistance that allow them to circumvent the innate immunity of their hosts. One example of these human infectious subspecies is T. b. rhodesiense, which circumvents TLF-1 killing and survives in the mammalian bloodstream through the expression of the Serum Resistance Associated protein (SRA). In this study, I have defined the morphologies of TLF-1 mediated cell lysis and demonstrated that each of the lytic components of TLF-1 gives rise to a specific cell death morphology, that is likely to be due to different mechanisms of trypanosome killing. I have also identified the post-translational modifications of SRA and defined its trafficking pathway from synthesis to degradation. SRA is GPI-anchored and has an unusual localization as a resident endosomal protein, before it is ultimately targeted to the lysosomal compartment and rapidly degraded. In addition, I have identified the initial point of colocalization of SRA and TLF-1 as the early endosomal compartment and demonstrated that TLF-1 appears to be less stable in the presence of SRA. Previous work by our group proposed that TLF-1 interacts with the negatively charged lysosomal membrane in order to mediate lysis. This interaction occurs through membrane insertion of the TLF-1 protein, apolipoprotein L-I, which also binds SRA. The mechanism of resistance is proposed to involve the interaction of SRA and TLF-1 through the binding of ApoL-I, which blocks TLF-1 association with the lysosomal membrane. Failure of TLF-1 to associate with the membrane leaves the toxin exposed to proteolysis in the lysosomal lumen and subsequent accelerated degradation. Thus, the mechanism of T. b. rhodesiense resistance to TLF-1 killing is likely to be a two-step process that blocks the localization of the trypanosome toxin from its required site of action, followed by clearance through lysosomal degradation.