| To gain insight into the physiological functions of APP family proteins, including APP, APLP1 and APLP2, a functional proteomic approach was used to identify proteins that interact with a highly conserved C-terminal region of APLP1. This approach identified two novel APP interacting proteins: ubiquitous mitochondrial creatine kinase (uMtCK) and 4-nitrophenylphosphatase domain and non-neuronal SNAP25-like protein homolog 1 (NIPSNAP1). The interactions of these two proteins with the C-terminal fragment and/or full-length APP family proteins were confirmed subsequently in cultured cells and/or in brain.;uMtCK is a well-studied mitochondrial enzyme. It catalyzes the reversible transfer of a high energy phosphate group from ATP to creatine to yield phosphocreatine and plays an important role in the energy metabolism and homeostasis. NIPSNAP1, a novel protein with unknown function, was shown to be localized in mitochondria by biochemical fractionation and immunofluorescence experiments in this study.;Both uMtCK and NIPSNAP1 are mitochondrial proteins with an N-terminal signal peptide. The preprotein forms, containing the signal peptide, exist in the cytosol and the mature forms in the mitochondria. Expression of APP C-terminus in cultured cells stabilized the preprotein forms of uMtCK and NIPSNAP1. The APP C-terminus also seemed to facilitate the uMtCK preprotein import into the mitochondria. Furthermore, the APP C-terminus was found to interact with TOM22, a receptor of mitochondrial import machinery.;Together, the findings that the APP C-terminus interacts with multiple mitochondrial proteins and stabilizes their unprocessed forms and that APP C-terminus interacts with TOM22 suggest that APP may regulate mitochondria functions, possibly as a mitochondrial chaperone protein. This study provides new insights not only into the physiological functions of APP proteins, but also into their potential pathological roles. |
