SORLA/SORL1 is a unique neuronal sorting receptor for the amyloid precursor protein that has been causally implicated in both sporadic and autosomal dominant familial forms of Alzheimer's disease (AD). Brain concentrations of SORLA are inversely correlated with amyloid-β (Aβ) in mouse models and AD patients, suggesting that increasing expression of this receptor could be a therapeutic option for decreasing the amount of amyloidogenic products in affected individuals. We characterize a new mouse model in which SORLA is overexpressed, and show a decrease in Aβ concentrations in mouse brain. We trace the underlying molecular mechanism to the ability of this receptor to direct lysosomal targeting of nascent Aβ peptides. Aβ binds to the amino-terminal VPS10P domain of SORLA, and this binding is impaired by a familial AD mutation in SORL1. Thus, loss of SORLA's Aβ sorting function is a potential cause of AD in patients, and SORLA may be a new therapeutic target for AD drug development.

SusB, an 84-kDa ␣-glucoside hydrolase involved in the starch utilization system (sus) of Bacteroides thetaiotaomicron, belongs to glycoside hydrolase (GH) family 97. We have determined the enzymatic characteristics and the crystal structures in free and acarbose-bound form at 1.6 Å resolution. SusB hydrolyzes the ␣-glucosidic linkage, with inversion of anomeric configuration liberating the ␤-anomer of glucose as the reaction product. The substrate specificity of SusB, hydrolyzing not only ␣-1,4-glucosidic linkages but also ␣-1,6-, ␣-1,3-, and ␣-1,2-glucosidic linkages, is clearly different from other well known glucoamylases belonging to GH15. The structure of SusB was solved by the single-wavelength anomalous diffraction method with sulfur atoms as anomalous scatterers using an in-house x-ray source. SusB includes three domains as follows: the N-terminal, catalytic, and C-terminal domains. The structure of the SusB-acarbose complex shows a constellation of carboxyl groups at the catalytic center; Glu 532 is positioned to provide protonic assistance to leaving group departure, with Glu 439 and Glu 508 both positioned to provide base-catalyzed assistance for inverting nucleophilic attack by water. A structural comparison with other glycoside hydrolases revealed significant similarity between the catalytic domain of SusB and those of ␣-retaining glycoside hydrolases belonging to GH27, -36, and -31 despite the differences in catalytic mechanism. SusB and the other retaining enzymes appear to have diverged from a common ancestor and individually acquired the functional carboxyl groups during the process of evolution. Furthermore, sequence comparison of the active site based on the structure of SusB indicated that GH97 included both retaining and inverting enzymes.

Semaphorin axonal guidance factors are multifunctional proteins that play important roles in immune response, cancer cell proliferation, and organogenesis, making semaphorins and their signaling receptor plexins important drug targets for various diseases. However, the large and flat binding surface of the semaphorin-plexin interaction interface is difficult to target by traditional small-molecule drugs. Here, we report the discovery of a high-affinity plexin B1 (PlxnB1)-binding macrocyclic peptide, PB1m6 (K = 3.5 nM). PB1m6 specifically inhibited the binding of physiological ligand semaphorin 4D (Sema4D) in vitro and completely suppressed Sema4D-induced cell collapse. Structural studies revealed that PB1m6 binds at a groove between the fifth and sixth blades of the sema domain in PlxnB1 distant from the Sema4D-binding site, indicating the non-competitive and allosteric nature of the inhibitory activity. The discovery of this novel allosteric site can potentially be used to target plexin family proteins for the development of drugs that modulate semaphorin and plexin signaling.