Competitive Inhibition of the Endoplasmic Reticulum Signal Peptidase by Non-cleavable Mutant Preprotein Cargos

Upon translocation across the endoplasmic reticulum (ER) membrane, secretory proteins are proteolytically processed to remove their signal peptide by signal peptidase (SPase). This process is critical for subsequent folding, intracellular trafficking, and maturation of secretory proteins. Prokaryotic SPase has been shown to be a promising antibiotic target. In contrast, to date, no eukaryotic SPase inhibitors have been reported. Here we report that introducing a proline immediately following the natural signal peptide cleavage site not only blocks preprotein cleavage but also, in trans, impairs the processing and maturation of co-expressed preproteins in the ER. Specifically, we find that a variant preproinsulin, pPI-F25P, is translocated across the ER membrane, where it binds to the catalytic SPase subunit SEC11A, inhibiting SPase activity in a dose-dependent manner. Similar findings were obtained with an analogous variant of preproparathyroid hormone, demonstrating that inhibition of the SPase does not depend strictly on the sequence or structure of the downstream mature protein. We further show that inhibiting SPase in the ER impairs intracellular processing of viral polypeptides and their subsequent maturation. These observations suggest that eukaryotic SPases (including the human ortholog) are, in principle, suitable therapeutic targets for antiviral drug design.     

Repeat motif-containing regions within thyroglobulin

Thyroglobulin (precursor for thyroid hormone synthesis) is a large secreted glycoprotein comprising contiguous region I (multiple type-1 repeating units engaging the first ～1,191 residues, followed by a ～245-residue hinge region), regions II-III (multiple type-2 and 3 repeating units, comprising ～720 residues), and the C-terminal cholinesterase-like (ChEL) domain (～570 residues). A signal peptide attached to ChEL makes an independent secretory protein that binds to I-II-III, stabilizing it and rescuing the secretion of I-II-III that would otherwise be trapped in the endoplasmic reticulum (ER). In this study, we found that a signal peptide attached to regions II-III also makes for an efficient secretory protein that neither demonstrably interacts nor has its secretion enhanced by the presence of secretory ChEL. By contrast, region I, either with or without the hinge region, cannot be secreted on its own and remains in the ER where it is bound to ER chaperones BiP and GRP94. Whereas ChEL can rescue secretion of I-II-III, it can rescue I-II only very weakly, and region I not at all. Yet, ChEL begins to rescue region I in cells that also co-express secretory II-III. The data suggest that conformational maturation of region I is a limiting step in the thyroglobulin maturation process, and this step is facilitated by the presence of both regions II-III and ChEL. Mutations causing hypothyroidism might induce solely local/regional misfolding or may interfere more globally by impeding interactions between regions that are required for thyroglobulin secretion.     

An N-terminal Fragment of the Prion Protein Binds to Amyloid-β Oligomers and Inhibits Their Neurotoxicity in Vivo

A hallmark of Alzheimer disease (AD) is the accumulation of the amyloid-β (Aβ) peptide in the brain. Considerable evidence suggests that soluble Aβ oligomers are responsible for the synaptic dysfunction and cognitive deficit observed in AD. However, the mechanism by which these oligomers exert their neurotoxic effect remains unknown. Recently, it was reported that Aβ oligomers bind to the cellular prion protein with high affinity. Here, we show that N1, the main physiological cleavage fragment of the cellular prion protein, is necessary and sufficient for binding early oligomeric intermediates during Aβ polymerization into amyloid fibrils. The ability of N1 to bind Aβ oligomers is influenced by positively charged residues in two sites (positions 23–31 and 95–105) and is dependent on the length of the sequence between them. Importantly, we also show that N1 strongly suppresses Aβ oligomer toxicity in cultured murine hippocampal neurons, in a Caenorhabditis elegans-based assay, and in vivo in a mouse model of Aβ-induced memory dysfunction. These data suggest that N1, or small peptides derived from it, could be potent inhibitors of Aβ oligomer toxicity and represent an entirely new class of therapeutic agents for AD.     

Protein disulfide isomerase-like proteins play opposing roles during retrotranslocation

Misfolded proteins in the endoplasmic reticulum (ER) are retained in the organelle or retrotranslocated to the cytosol for proteasomal degradation. ER chaperones that guide these opposing processes are largely unknown. We developed a semipermeabilized cell system to study the retrotranslocation of cholera toxin (CT), a toxic agent that crosses the ER membrane to reach the cytosol during intoxication. We found that protein disulfide isomerase (PDI) facilitates CT retrotranslocation, whereas ERp72, a PDI-like protein, mediates its ER retention. In vitro analysis revealed that PDI and ERp72 alter CT's conformation in a manner consistent with their roles in retrotranslocation and ER retention. Moreover, we found that PDI's and ERp72's opposing functions operate on endogenous ER misfolded proteins. Thus, our data identify PDI family proteins that play opposing roles in ER quality control and establish an assay to further delineate the mechanism of CT retrotranslocation.