Roden, R. | Search bioRxiv

bioRxiv2026

RNA-binding is the essential biological function of the Drosophila protein Brat

Brain tumor (Brat) is a Drosophila TRIM-NHL protein required for embryogenesis and neural stem cell differentiation. Although structural and biochemical studies established that the Brat NHL domain specifically binds RNA, the in vivo requirement for this activity has not been directly tested. Here, we used structure-guided mutagenesis and genome engineering to determine whether RNA recognition is essential for Brat function during development. The direct interaction between Brats NHL domain and RNA containing Brat Binding Sites (BBS) can be abolished by alanine substitution of three separate residues on the NHL surface. We introduced these point mutations into the endogenous brat locus by CRISPR-mediated Scarless Gene Editing to generate three independent RNA-binding defective mutant (RBDmt) alleles. Complementation tests demonstrated that each allele behaves as a strong loss-of-function mutation: homozygotes and hemizygotes are inviable, and RBDmt alleles fail to complement classical brat null and hypomorphic alleles. Lethal phase analysis revealed death predominantly during late larval and pupal stages, consistent with known brat alleles. Consistent with the namesake brat phenotype, RBDmt larval brains exhibited widespread expression of neuroblast markers and a marked reduction of neuronal differentiation. In embryos, these alleles failed to complement female sterile brat alleles and recapitulated characteristic abdominal segmentation defects. Finally, RT-qPCR showed increased expression of endogenous Brat target mRNAs in mutant larvae, consistent with loss of Brat-mediated repression. Together, these results demonstrate that direct RNA binding is the essential molecular activity of Brat and that post-transcriptional regulation of Brat target mRNAs underlies its critical roles across development.

molecular biology↗

bioRxiv2021

Protein detection in blood with single-molecule imaging

The ability to identify and characterize individual biomarker protein molecules in patient blood samples could enable diagnosis of diseases at an earlier stage, when treatment is typically more effective. Single-molecule imaging offers a promising approach to accomplish this goal. However, thus far single-molecule imaging methods have only been used to monitor protein molecules in solutions or cell lysates, and have not been translated into the clinical arena. Furthermore, the detection limit of these methods has been confined to the picomolar (10-12 M) range. In many diseases, the circulating concentrations of biomarker proteins fall several orders of magnitude below this range. Here we describe Single-Molecule Augmented Capture (SMAC), a single-molecule imaging technique to visualize, quantify, and characterize individual protein molecules of interest down to the subfemtomolar (<10-15 M) range, even in complex biologic fluids. We demonstrate SMAC in a wide variety of applications with human blood samples, including the analysis of disease-associated secreted proteins, membrane proteins, and rare intracellular proteins. Using ovarian cancer as a model, a lethal malignancy in which high-grade disease is driven almost universally by alterations in the TP53 gene and frequently only diagnosed at a late, incurable stage, we found that mutant pattern p53 proteins are released into the blood in patients at an early stage in disease progression. SMAC opens the door to the application of single-molecule imaging in non-invasive disease profiling and allows for the analysis of circulating mutant proteins as a new class of highly specific disease biomarkers. The SMAC platform can be adapted to multiplex or high-throughput formats to characterize heterogeneous biochemical and structural features of circulating proteins-of-interest. One Sentence SummaryA single-molecule imaging approach detects individual disease-specific protein molecules, including mutant intracellular proteins, in blood samples.

biophysics↗