Stuctural and functional analysis of signal peptide peptidase-like (SPPL) proteasemediated intramembrane proteolysis
As the major type of dementia in the elderly, Alzheimerʻs disease (AD) is a neurodegenerative disorder associated with progressive and irreversible loss of cognitive capabilities and memory. Histologically, AD is characterized by the presence of amyloid plaques and neurofibrillary tangles within patientsʻ brain samples. Amyloid plaques are large proteinaceous aggregates of amyloid β (Aβ) peptides and generation of these Aβ peptides is implicated to trigger a cascade of molecular events leading to neuronal cell death and eventually clinically apparent AD.
Aβ peptides are generated by stepwise proteolysis of the amyloid precursor precursor protein (APP). APP is initially processed by the β-secretase BACE and Aβ peptides are released from the cell membrane by subsequent intramembrane proteolysis mediated by the γ-secretase complex. Presenilins (PS), the active protein components of the γ-secretase complex, belong to a family of intramembrane-cleaving aspartyl proteases. This family, termed the GxGD-type aspartyl proteases due to a conserved active site motif, also comprises the signal peptide peptidase (SPP) and SPP-like proteases. Given the role of the γ-secretase complex in AD pathophysiology, this protease family is of particular interest as a very promising therapeutic target.
The project will focus on the elucidation of the atomic structure of human SPPL3, a prototypic and structurally less complex member of this family, by X-ray crystallography. This will provide insight into the catalytic and mechanistic features of this protease family and given its homologous biology to PS, it will be valuable for future drug development efforts. In addition, the determinants of substrate selectivity of the family members will be analyzed by mutagenesis approaches. Strikingly, unlike PS, SPP/SPPLs are not endoproteolysed and this discrepancy will also be addressed experimentally. Hence, this work will provide critical insight into general principles of intramembrane proteolysis by GxGD-type aspartyl proteases and will contribute to our current understanding of initial events of AD pathology.