A single exon-encoded Theileria parva strain Muguga cysteine protease (ThpCP): Molecular modelling and characterisation.

The tick-transmitted apicomplexan Theileria parva causes East Coast fever, a bovine disease of great economic and veterinary importance in Africa. Papain-like cysteine proteases play important roles in protozoan parasite host cell entry and egress, nutrition and host immune evasion. This study reports the identification and characterisation of a T. parva strain Muguga cathepsin L-like (C1A subfamily) cysteine protease (ThpCP). Molecular modelling confirmed the papain-like fold of ThpCP, hydrophobic character of the S2 substrate binding pocket and non-covalent interaction between the pro- and catalytic domains preceding low pH autoactivation. ThpCP was recombinantly expressed in a protease deficient E. coli (Rosetta (DE3)pLysS strain) expression host as a 46 kDa proenzyme. Following Ni-chelate affinity chromatography and acidification, the 27 kDa mature ThpCP was purified by cation-exchange chromatography. Purified ThpCP hydrolysed typical cathepsin L substrates N-α-benzyloxycarbonyl (Z)-Phe-Arg-7-amino-4-methyl-coumarin (AMC) (kcat/Km = 4.49 × 105 s-1M-1) and Z-Leu-Arg-AMC (kcat/Km = 4.20 × 105 s-1M-1), but showed no activity against the cathepsin B-selective substrate Z-Arg-Arg-AMC. Recombinant ThpCP was active over a broad pH range from pH 4.5 to 7.5, thereby showing potential activity in the acidic parasite food vacuole and close to neutral pH of the host lymphocyte cytoplasm. Recombinant ThpCP was inhibited by the cysteine protease inhibitors E64, iodoacetate, leupeptin, chymostatin, Z-Phe-Ala-diazomethylketone (DMK) and Z-Phe-Phe-DMK and hydrolysed bovine proteins: haemoglobin, immunoglobulin G, serum albumin and fibrinogen as well as goat IgG at pH 6 and 7. Functional expression and characterisation of Theileria cysteine proteases should enable high throughput screening of cysteine protease inhibitor libraries against these proteases.