Thermobifida fusca o-succinylbenzoate synthase (OSBS), a member of the enolase superfamily that catalyzes a step in menaquinone biosynthesis, shares 22% and 28% amino acid sequence identity with two previously characterized OSBS enzymes from Escherichia coli and Amycolatopsis sp. T-1-60, respectively. These values are considerably lower than typical sequence identities among homologous proteins that have the same function. To determine how such divergent enzymes catalyze the same reaction, we solved the structure of T. fusca OSBS and identified amino acids that are important for ligand binding. We discovered significant differences in structure and conformational flexibility between T. fusca OSBS and other members of the enolase superfamily. In particular, the 20s loop, a flexible loop in the active site that permits ligand binding and release in most enolase superfamily proteins, has a four-amino acid deletion and is well ordered in T. fusca OSBS. Instead, flexibility of a different region allows the substrate to enter from the other side of the active site. T. fusca OSBS was more tolerant of mutations at residues that were critical for activity in E. coli OSBS. Also, replacing active site amino acids found in one protein with the amino acids that occur at the same place in the other protein reduces catalytic efficiency. Thus, the extraordinary divergence between these proteins does not appear to reflect a higher tolerance of mutations. Instead, large deletions outside the active site were accompanied by alteration of active site size and electrostatic interactions, resulting in small but significant differences in ligand binding.

Tauopathies have diverse presentation, progression, and neuropathology. They are linked to tau prion strains, self-replicating assemblies of unique quaternary conformation. Strains can be propagated indefinitely in cultured cells, and induce unique patterns of transmissible neuropathology upon inoculation into mice. Aggregates from a single strain reproduce only that strain upon re-inoculation into cells or mice. DS9 and DS10 cell lines propagate distinct synthetic strains. Surprisingly, DS9 monomer inoculated into naïve cells encoded an identical "sub-strain," whereas DS10 monomer encoded multiple sub-strains. Sub-strains produced distinct pathology upon inoculation into a tauopathy mouse model (PS19). Brainderived tau monomer from an Alzheimer's brain encoded a single strain. Monomer from a corticobasal degeneration brain encoded three sub-strains in which monomer from each encoded all three upon re-inoculation into cells. Tau monomer thus adopts multiple, stable seed-competent conformations, each of which encodes a limited number of strains. This provides insights into the origins of distinct tauopathies.

PurposeRetinitis pigmentosa (RP) is an inherited retinal disorder that results in the degeneration of photoreceptor cells, ultimately leading to severe visual impairment. We characterized a consanguineous family from Southern India wherein an individual in his 20’s presented with night blindness since childhood. The purpose of this study was to identify the causative mutation for RP in this individual as well as characterize how the mutation may ultimately affect protein function.MethodsWe performed a complete ophthalmologic examination of the proband followed by exome sequencing. The identified mutation was then modeled in cultured cells, evaluating its expression, solubility (both by western blot), subcellular distribution (confocal microscopy), and testing whether this variant induced endoplasmic reticulum (ER) stress (qPCR and western blotting).ResultsThe proband presented with generalized and parafoveal retinal pigment epithelial atrophy with bone spicule pigmentation in the mid periphery and arteriolar attenuation. Optical coherence tomography scans through the macula of both eyes showed atrophy of outer retinal layers with loss of the ellipsoid zone, whereas systemic examination of this individual was normal. The proband’s parents and sibling were asymptomatic and had normal funduscopic examinations. We discovered a novel homozygous p.Pro388Ser mutation in the tubby-like protein 1 (TULP1) gene in the individual with RP. In cultured cells, the P388S mutation does not alter the subcellular distribution of TULP1 or induce ER stress when compared to wild-type TULP1, but instead significantly lowers protein stability as indicated by steady-state and cycloheximide-chase experiments.ConclusionsThese results add to the list of known TULP1 mutations associated with RP and suggest a unique pathogenic mechanism in TULP1-induced RP, which may be shared amongst select mutations in TULP1.

Fibulin-3 (F3 or EFEMP1) is a disulfide-rich, secreted glycoprotein necessary for maintaining extracellular matrix (ECM) and connective tissue integrity. Two studies have identified distinct autosomal recessive F3 mutations in individuals with Marfan Syndrome-like phenotypes. Herein we characterized how one of these mutations, c.163T>C; p.Cys55Arg (C55R), disrupts F3 secretion, quaternary structure, and function by forming unique extracellular disulfide-linked homodimers. Dual cysteine mutants suggest that the C55R-induced disulfide species forms because of new availability of Cys70 on adjacent F3 monomers. Surprisingly, mutation of single cysteines located near C55 (i.e., C29, C42, C48, C61, C70, C159, and C171) also produced similar extracellular disulfide-linked dimers, suggesting that this is not a phenomenon isolated to the C55R mutant. To assess C55R functionality, F3 knockout (KO) retinal pigmented epithelial (RPE) were generated, followed by reintroduction of wild-type (WT) or C55R F3. F3 KO cells produced lower levels of the ECM remodeling enzyme, matrix metalloproteinase 2 and reduced formation of collagen VI ECM filaments, both of which were partially rescued by WT F3 overexpression. However, C55R F3 was unable to compensate for these same ECM-related defects. Our results highlight the unique behavior of particular cysteine mutations in F3 and uncover potential routes to restore C55R F3 loss-of-function.

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The Escherichia coli dihydrofolate reductase (DHFR) destabilizing domain (DD) serves as a promising approach to conditionally regulate protein abundance in a variety of tissues. In the absence of TMP, a DHFR stabilizer, the DD is degraded by the ubiquitin proteasome system (UPS). To test whether this approach could be effectively applied to a wide variety of aged and disease-related ocular mouse models, which may have a compromised UPS, we evaluated the DHFR DD system in aged mice (up to 24 mo), a light-induced retinal degeneration (LIRD) model, and two genetic models of retinal degeneration (rd2 and Abca4-/- mice). Aged, LIRD, and Abca4-/- mice all had similar proteasomal activities and high-molecular weight ubiquitin levels compared to control mice. However, rd2 mice displayed compromised chymotrypsin activity compared to control mice. Nonetheless, the DHFR DD was effectively degraded in all model systems, including rd2 mice. Moreover, TMP increased DHFR DD-dependent retinal bioluminescence in all mouse models, however the fold induction was slightly, albeit significantly, lower in Abca4-/- mice. Thus, the destabilized DHFR DD-based approach allows for efficient control of protein abundance in aged mice and retinal degeneration mouse models, laying the foundation to use this strategy in a wide variety of mice for the conditional control of gene therapies to potentially treat multiple eye diseases.

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