Proto, William Richard
Characterisation of autophagy and a metacaspase in Trypanosoma brucei.
PhD thesis, University of Glasgow.
Full text available as:
This project focuses on the characterisation of two separate aspects of Trypanosoma brucei cell biology; the degradative process of autophagy and a specific cysteine peptidase from the metacaspase family.
Autophagy is a widely conserved intracellular mechanism for the degradation of long lived proteins and organelles, that requires the formation of an autophagosome (double membrane bound vesicle) around cargo destined for the lysosome. The molecular machinery involved in autophagy has been well characterised in yeast and bioinformatic screens have identified many of the core components in T. brucei. However, beyond bioinformatics there is limited experimental evidence to support the presence of functional autophagy in T. brucei.
A key component of the autophagic pathway is ATG8, a ubiquitin-like protein that is incorporated into the autophagosome membrane. To investigate autophagy in T. brucei the three candidate ATG8 genes (ATG8.1 Tb927.7.5900, ATG8.2 Tb927.7.5910 and ATG8.3 Tb927.7.3320) were fused with yellow fluorescent protein (YFP) and expressed in bloodstream form and procyclic form T. brucei cultured parasites. Fluorescent microscopy was used to monitor the formation of YFP-labelled autophagosomes, which enabled the evaluation of the autophagic response towards a variety of different stimuli. We provide the first direct experimental evidence confirming a functional autophagy pathway in T. brucei and show that it is induced in response to nutrient starvation in the procyclic form and neuropeptide treatment in the bloodstream form and can be blocked by the classical autophagy inhibitor wortmannin. Characterisation of the T. brucei ATG8 family revealed that ATG8.1 and ATG8.2 appear to operate as ‘ATG8-like’ proteins, whereas ATG8.3 behaves atypically, possibly functioning as an ATG12 protein. Furthermore, targeted RNAi downregulation of the predicted T. brucei ATG3 (Tb927.2.1890) caused a reduction in cell growth. The vital role of ATG3 in the autophagy pathway suggested that the process was required for normal procyclic form growth.
The second focus of the project was metacaspase 4 (Tb927.10.2440) which belongs to the metacaspases (MCAs), cysteine peptidases of the caspase family found in plants, fungi and protozoa, but absent from mammals. Of the five MCAs possessed by T. brucei, only MCA2, MCA3 and MCA5 contain the conserved histidine cysteine catalytic dyad. MCA1 and MCA4 are predicted to contain key substitutions within their active sites, raising interesting questions regarding potential peptidase activity and functions. The exact role of the T. brucei metacaspases remains largely unknown, with MCA2 , MCA3 and MCA5 appearing to function in association with RAB11 positive endosomes, although independently of the known recycling functions of these endosomes (Helms et al. 2006).
To develop our understanding of the MCA family in T. brucei a study into the function of MCA4 was undertaken. An antibody was raised against MCA4 and western blotting of cell lysate revealed that MCA4 expression occurred only in bloodstream form T. brucei. Interestingly MCA4 localised to the flagellar membrane, appearing in a linear array of punctate structures. Dual acylation is known to mediate flagellar membrane association in T. brucei and was implicated in MCA4 targeting following bioinformatic predictions and subsequent experimental confirmation of MCA4 palmitoylation using an acyl-biotin exchange reaction.
MCA4 contains a non-canonical active site residue (serine-219) in the position of the predicted conserved active site cysteine. Activity assays using purified recombinant protein revealed that full length MCA4 was unable to autoprocess and was inactive. However, MCA4 peptidase activity could be detected following proteolytic activation with MCA2. Interestingly, removal of the MCA4 active site serine by mutagenesis (MCA4S219G) did not abolish activity of the processed enzyme, revealing an alternative nucleophile was capable of contributing to activity. Furthermore, mutation of the active site serine to cysteine, produced a constitutively active peptidase capable of autolytic processing in a calcium dependent manner.
Following these key findings the role of MCA4 in the T. brucei lifecycle was investigated by RNAi and genetic knockout. Rapid depletion of MCA4 by RNAi caused a block in cytokinesis followed by cell death, nevertheless the generation of MCA4 null mutant parasites (∆mca4) was possible. A role for MCA4 in mammalian infection was revealed by monitoring infection progression in mice. Deletion of MCA4 increased host survival and parasite virulence could be restored by the ectopic re-expression of MCA4 in ∆mca4 parasites.
Actions (login required)