The major secreted cathepsin L1 protease of the liver fluke, Fasciola hepatica -: A Leu-12 to Pro-12 replacement in the nonconserved C-terminal region of the prosegment prevents complete enzyme autoactivation and allows definition of the molecular events in prosegment removal

A protease secreted by the parasitic helminth Fasciola hepatica, a 37-kDa procathepsin L1 (FheproCL1), autocatalytically processes and activates to its mature enzyme (FheCL1) over a wide pH range of 7.3 to 4.0, although activation is more rapid at low pH. Maturation initiates with cleavages of a small proportion of molecules within the central region of the prosegment, possibly by intramolecular events. However, activation to fully mature enzymes is achieved by a precise intermolecular cleavage at a Leu (-12)-Ser(-11) down arrow His(-)10 sequence within the nonconserved C-terminal region of the prosegment. The importance of this cleavage site in enzyme activation was demonstrated using an active site variant FheproCL1Gly(26) ( Cys(26) to Gly(26)) and a double variant FheproCL1Pro(-12)/Gly(26) ( Leu (-)12 to Pro(-12)), and although both of these variants cannot autocatalytically process, the former is susceptible to trans-processing at a Leu(-12)-Ser-11 down arrow His-10 sequence by pre- activated FheCL1, but the latter is not. Another F. hepatica secreted protease FheCL2, which, unlike FheCL1, can readily accept proline in the S2 subsite of its active site, can trans-process the double variant FheproCL1Pro(-12)/Gly(26) by cleavage at the Pro (-12) -Ser(-11 down arrow)2His(-10) sequence. Furthermore, the autoactivation of a variant enzyme with a single replacement, FheproCL1Pro(-12), was very slow but was increased 40-fold in the presence of FheCL2. These studies provide a molecular insight into the regulation of FheproCL1 autocatalysis.