Karan Mahe

Bio: Karan Mahe is an academic researcher from University of California, Davis. The author has contributed to research in topics: Centrosome & PCNT. The author has an hindex of 3, co-authored 7 publications receiving 52 citations. Previous affiliations of Karan Mahe include University of California, Berkeley.

Co-translational protein targeting facilitates centrosomal recruitment of PCNT during centrosome maturation in vertebrates

Abstract: As microtubule-organizing centers of animal cells, centrosomes guide the formation of the bipolar spindle that segregates chromosomes during mitosis. At mitosis onset, centrosomes maximize microtubule-organizing activity by rapidly expanding the pericentriolar material (PCM). This process is in part driven by the large PCM protein pericentrin (PCNT), as its level increases at the PCM and helps recruit additional PCM components. However, the mechanism underlying the timely centrosomal enrichment of PCNT remains unclear. Here, we show that PCNT is delivered co-translationally to centrosomes during early mitosis by cytoplasmic dynein, as evidenced by centrosomal enrichment of PCNT mRNA, its translation near centrosomes, and requirement of intact polysomes for PCNT mRNA localization. Additionally, the microtubule minus-end regulator, ASPM, is also targeted co-translationally to mitotic spindle poles. Together, these findings suggest that co-translational targeting of cytoplasmic proteins to specific subcellular destinations may be a generalized protein targeting mechanism.

Condensation of pericentrin proteins in human cells illuminates phase separation in centrosome assembly

Abstract: At the onset of mitosis, centrosomes expand the pericentriolar material (PCM) to maximize their microtubule-organizing activity. This step, termed centrosome maturation, ensures proper spindle organization and faithful chromosome segregation. However, as the centrosome expands, how PCM proteins are recruited and held together without membrane enclosure remains elusive. We found that endogenously expressed pericentrin (PCNT), a conserved PCM scaffold protein, condenses into dynamic granules during late G2/early mitosis before incorporating into mitotic centrosomes. Furthermore, the N-terminal portion of PCNT, enriched with conserved coiled-coils (CCs) and low-complexity regions (LCRs), phase separates into dynamic condensates that selectively recruit PCM proteins and nucleate microtubules in cells. We propose that CCs and LCRs, two prevalent sequence features in the centrosomal proteome, are preserved under evolutionary pressure in part to mediate liquid-liquid phase separation, a process that bestows upon the centrosome distinct properties critical for its assembly and functions.

Condensation of pericentrin proteins in human cells illuminates phase separation in centrosome assembly

Abstract: Mitotic centrosomes are complex membraneless organelles that guide the formation of mitotic spindles to ensure faithful cell division. They are formed by timely expansion of the pericentriolar material (PCM) around the centrioles at the onset of mitosis. How PCM proteins are recruited and held together without a lipid membrane remains elusive. Here we found that endogenously expressed pericentrin (PCNT), a conserved PCM scaffold protein, condenses into liquid-like granules during early mitosis in cultured human cells. Furthermore, the N-terminal segment of PCNT, enriched with conserved coiled-coils and low-complexity regions (LCRs), undergoes phase separation. These PCNT 9condensates9 selectively recruit PCM components and nucleate microtubules in cells. We propose that coiled-coils and LCRs, two prevalent sequence features in the centrosomal proteome, are preserved under evolutionary pressure to drive phase separation, a process that bestows upon the centrosome a distinct material property critical for its assembly and functions.

Co-translational protein targeting facilitates centrosomal recruitment of PCNT during centrosome maturation

Abstract: As microtubule-organizing centers of animal cells, centrosomes guide the formation of the bipolar spindle that segregates chromosomes during mitosis At mitosis onset, centrosomes maximize microtubule-organizing activity by rapidly expanding the pericentriolar material (PCM) This process is in part driven by the large PCM protein pericentrin (PCNT), as its level increases at the PCM and helps recruit additional PCM components However, the mechanism underlying the timely centrosomal enrichment of PCNT remains unclear Here we show that PCNT is delivered co-translationally to centrosomes during early mitosis by cytoplasmic dynein, as evidenced by centrosomal enrichment of PCNT mRNA, its translation near the centrosome, and requirement of intact polysomes for PCNT mRNA localization Additionally, the microtubule minus-end regulator, ASPM, is also targeted co-translationally to mitotic spindle poles Together, these findings suggest that co-translational targeting of cytoplasmic proteins to specific subcellular destinations may be a generalized protein targeting mechanism

Computational Design of Virus-Like Protein Assemblies on Carbon

Abstract: Proteins are designed to bind to specific surfaces while also presenting a programmed surface superstructure. There is a general need for the engineering of protein-like molecules that organize into geometrically specific superstructures on molecular surfaces, directing further functionalization to create richly textured, multilayered assemblies. Here we describe a computational approach whereby the surface properties and symmetry of a targeted surface define the sequence and superstructure of surface-organizing peptides. Computational design proceeds in a series of steps that encode both surface recognition and favorable intersubunit packing interactions. This procedure is exemplified in the design of peptides that assemble into a tubular structure surrounding single-walled carbon nanotubes (SWNTs). The geometrically defined, virus-like coating created by these peptides converts the smooth surfaces of SWNTs into highly textured assemblies with long-scale order, capable of directing the assembly of gold nanoparticles into helical arrays along the SWNT axis.

Intracellular mRNA transport and localized translation

Abstract: Fine-tuning cellular physiology in response to intracellular and environmental cues requires precise temporal and spatial control of gene expression. High-resolution imaging technologies to detect mRNAs and their translation state have revealed that all living organisms localize mRNAs in subcellular compartments and create translation hotspots, enabling cells to tune gene expression locally. Therefore, mRNA localization is a conserved and integral part of gene expression regulation from prokaryotic to eukaryotic cells. In this Review, we discuss the mechanisms of mRNA transport and local mRNA translation across the kingdoms of life and at organellar, subcellular and multicellular resolution. We also discuss the properties of messenger ribonucleoprotein and higher order RNA granules and how they may influence mRNA transport and local protein synthesis. Finally, we summarize the technological developments that allow us to study mRNA localization and local translation through the simultaneous detection of mRNAs and proteins in single cells, mRNA and nascent protein single-molecule imaging, and bulk RNA and protein detection methods.

Mapping spatial transcriptome with light-activated proximity-dependent RNA labeling.

Abstract: RNA molecules are highly compartmentalized in eukaryotic cells, with their localizations intimately linked to their functions. Despite the importance of RNA targeting, our current knowledge of the spatial organization of the transcriptome has been limited by a lack of analytical tools. In this study, we develop a chemical biology approach to label RNAs in live cells with high spatial specificity. Our method, called CAP-seq, capitalizes on light-activated, proximity-dependent photo-oxidation of RNA nucleobases, which could be subsequently enriched via affinity purification and identified by high-throughput sequencing. Using this technique, we investigate the local transcriptomes that are proximal to various subcellular compartments, including the endoplasmic reticulum and mitochondria. We discover that messenger RNAs encoding for ribosomal proteins and oxidative phosphorylation pathway proteins are highly enriched at the outer mitochondrial membrane. Due to its specificity and ease of use, CAP-seq is a generally applicable technique to investigate the spatial transcriptome in many biological systems.