The Structure of Human Tripeptidyl Peptidase II as Determined by a Hybrid Approach

Summary Tripeptidyl-peptidase II (TPPII) is a high molecular mass (∼5 MDa) serine protease, which is thought to act downstream of the 26S proteasome, cleaving peptides released by the latter. Here, the structure of human TPPII ( Hs TPPII) has been determined to subnanometer resolution by cryoelectron microscopy and single-particle analysis. The complex is built from two strands forming a quasihelical structure harboring a complex system of inner cavities. Hs TPPII particles exhibit some polymorphism resulting in complexes consisting of nine or of eight dimers per strand. To obtain deeper insights into the architecture and function of Hs TPPII, we have created a pseudoatomic structure of the Hs TPPII spindle using a comparative model of Hs TPPII dimers and molecular dynamics flexible fitting. Analyses of the resulting hybrid structure of the Hs TPPII holocomplex provide new insights into the mechanism of maturation and activation.