Hollow Octadecameric Self-Assembly of Collagen-like Peptides.

The folding of collagen is a hierarchical process that starts with three peptides associating into the characteristic triple helical fold. Depending on the specific collagen in question, these triple helices then assemble into bundles reminiscent of α-helical coiled-coils. Unlike α-helices, however, the bundling of collagen triple helices is very poorly understood with almost no direct experimental data available. In order to shed light on this critical step of collagen hierarchical assembly, we have examined the collagenous region of complement component 1q. Thirteen synthetic peptides were prepared to dissect the critical regions allowing for its octadecameric self-assembly. We find that short peptides (under 40 amino acids) are able to self-assemble into specific (ABC)6 octadecamers. This requires the ABC heterotrimeric composition as the self-assembly subunit, but does not require disulfide bonds. Self-assembly into this octadecamer is aided by short noncollagenous sequences at the N-terminus, although they are not entirely required. The mechanism of self-assembly appears to begin with the very slow formation of the ABC heterotrimeric helix, followed by rapid bundling of triple helices into progressively larger oligomers, terminating in the formation of the (ABC)6 octadecamer. Cryo-electron microscopy reveals the (ABC)6 assembly as a remarkable, hollow, crown-like structure with an open channel approximately 18 Å at the narrow end and 30 Å at the wide end. This work helps to illuminate the structure and assembly mechanism of a critical protein in the innate immune system and lays the groundwork for the de novo design of higher order collagen mimetic peptide assemblies.

Beyond the Triple Helix: Exploration of the Hierarchical Assembly Space of Collagen-like Peptides.

The de novo design of self-assembling peptides has garnered significant attention in scientific research. While alpha-helical assemblies have been extensively studied, exploration of polyproline type II (PPII) helices, such as those found in collagen, remains relatively limited. In this study, we focused on understanding the sequence-structure relationship in hierarchical assemblies of collagen-like peptides, using defense collagen SP-A as a model. By dissecting the sequence derived from SP-A and synthesizing short collagen-like peptides, we successfully constructed a discrete bundle of hollow triple helices. Mutation studies pinpointed amino acid sequences, including hydrophobic and charged residues that are critical for oligomer formation. These insights guided the de novo design of collagen-like peptides, resulting in the formation of diverse quaternary structures, including discrete and heterogenous bundled oligomers, 2D nanosheets, and pH-responsive nanoribbons. Our study represents a significant advancement in the understanding and harnessing of collagen higher-order assemblies beyond the triple helix.

Mitochondria-affecting small molecules ameliorate proteostasis defects associated with neurodegenerative diseases.

Macroautophagic recycling of dysfunctional mitochondria, known as mitophagy, is essential for mitochondrial homeostasis and cell viability. Accumulation of defective mitochondria and impaired mitophagy have been widely implicated in many neurodegenerative diseases, and loss-of-function mutations of PINK1 and Parkin, two key regulators of mitophagy, are amongst the most common causes of heritable parkinsonism. This has led to the hypothesis that pharmacological stimulation of mitophagy may be a feasible approach to combat neurodegeneration. Toward this end, we screened ~ 45,000 small molecules using a high-throughput, whole-organism, phenotypic screen that monitored accumulation of PINK-1 protein, a key event in mitophagic activation, in a Caenorhabditis elegans strain carrying a Ppink-1::PINK-1::GFP reporter. We obtained eight hits that increased mitochondrial fragmentation and autophagosome formation. Several of the compounds also reduced ATP production, oxygen consumption, mitochondrial mass, and/or mitochondrial membrane potential. Importantly, we found that treatment with two compounds, which we named PS83 and PS106 (more commonly known as sertraline) reduced neurodegenerative disease phenotypes, including delaying paralysis in a C. elegans ß-amyloid aggregation model in a PINK-1-dependent manner. This report presents a promising step toward the identification of compounds that will stimulate mitochondrial turnover.