Functional analysis of a family of proteins implicated in trypanosoma brucei lifecycle progression

Abstract

Bloodstream trypanosomes initiate differentiation to procyclic forms in response to a citrate/ cis-aconitate (CCA) signal. A cell line was previously selected (“defective in differentiation-clone 1”; DiD1) that was unable to differentiate to procyclic forms (Tasker et al. (2000)). Additionally, expression profiling of this line in comparison to the parental line by macroarray hybridisation identified two differentially-expressed transcripts from an 8 gene cluster of highly homologous genes we named PAD genes (Proteins Associated with Differentiation). Members of this family show distinct expression profiles throughout the trypanosome lifecycle at both the mRNA and protein level, and are localised to the cell surface membrane of the cell. At least 1 member of the family (PAD1) shows stumpy form specific RNA and protein expression, representing the first useful molecular marker for this stage, and exhibits biochemical specificity for citrate. Additionally, another member of this family (PAD2) is upregulated in response to low temperature, a condition reported to cause hypersensitivity to CCA. Finally, RNAi mediated ablation of the PAD gene transcripts compromised the capacity of stumpy form trypanosomes to differentiate to the procyclic form in response to CCA. These combined expression, cytological, reverse-genetic and biochemical data make PAD proteins excellent candidates for recognition of the signal to initiate differentiation in response to CCA.