C-terminal domain of human pancreatic lipase is required for stability and maximal activity but not colipase reactivation

Fungal lipases and human pancreatic lipase (hPL) share a common tertiary structure termed the α/β hydrolase fold. In contrast, the region C-terminal to the common tertiary structure does not share any common structural features with fungal lipases, leading to the hypothesis that the divergent C- terminal domain confers specific properties to hPL. To study the role of the C-terminal domain of hPL function, we made substitution and deletion mutations in the C-terminal domain. The mutant proteins were expressed in transfected COS-1 cells and the secreted proteins were analyzed by immunoblot and for lipase activity. Substitution mutants in multiple lysine residues, in aspartate 390, or in tyrosine 404 did not affect secretion or lipase activity of the mutants. Significantly, the mutants still required colipase for maximal activity. Deletion of the C-terminal domain decreased the amount of truncated, mutant protein in the medium of transfected cells and decreased the specific activity of the mutants. Still, maximal activity required colipase, indicating that the deletion mutants interacted with colipase. Interfacial binding of the truncated deletion mutants was decreased relative to wild-type hPL. The newly synthesized deletion mutants were not as efficiently secreted from the transfected cells as wild-type hPL, and the mutant proteins that appeared in the medium were less stable than the wild- type hPL. These findings suggest that the C-terminal domain is required for proper folding or processing of hPL, confers stability, and increases activity, but is not absolutely required for colipase reactivation of the bile salt-inhibited enzyme.