Mapping the conformational space accessible to BACE2 using surface mutants and cocrystals with Fab fragments, Fynomers and Xaperones

The aspartic protease BACE2 is responsible for the shedding
of the transmembrane protein Tmem27 from the surface of
pancreatic b-cells, which leads to inactivation of the b-cell
proliferating activity of Tmem27. This role of BACE2 in the
control of b-cell maintenance suggests BACE2 as a drug target
for diabetes. Inhibition of BACE2 has recently been shown
to lead to improved control of glucose homeostasis and to
increased insulin levels in insulin-resistant mice. BACE2 has
52% sequence identity to the well studied Alzheimer's disease
target enzyme b-secretase (BACE1). High-resolution BACE2
structures would contribute significantly to the investigation
of this enzyme as either a drug target or anti-target. Surface
mutagenesis, BACE2-binding antibody Fab fragments, single-
domain camelid antibody VHH fragments (Xaperones) and
Fyn-kinase-derived SH3 domains (Fynomers) were used as
crystallization helpers to obtain the first high-resolution
structures of BACE2. Eight crystal structures in six different
packing environments define an ensemble of low-energy
conformations available to the enzyme. Here, the different
strategies used for raising and selecting BACE2 binders for
cocrystallization are described and the crystallization success,
crystal quality and the time and resources needed to obtain
suitable crystals are compared.