Biology subjects

Serrao, V. H. B.

Publications and source records attributed to Serrao, V. H. B..

4 recordsLinked to original sources

bioRxiv2026

Diel remodeling and cellular integration of the nitroplast

Nitrogen-fixing eukaryotes were not believed to exist in nature until the recent discovery of a N2-fixing organelle, or nitroplast, in the marine microalga Braarudosphaera bigelowii. This nitroplast (formerly known as UCYN-A2) has long been recognized as key cyanobacterial contributor to global oceanic N2 fixation. However, how this novel organelle is integrated and regulated within the architecture of a eukaryotic cell remains unclear. Here, we combine multiscale volumetric imaging with cryo-electron tomography to resolve the native architecture, cellular integration, and diel remodeling of the nitroplast in cultured and environmental cells. We find that the nitroplast occupies up to 10% of the cell volume and exhibits close interfaces with multiple host organelles through membrane contact sites, while integration of this metabolically demanding compartment does not disrupt global scaling of host organelles. Interestingly, the chloroplast-to-nitroplast volume ratio is conserved across distinct life stages. Cryo-electron tomography reveals that the nitroplast retains a reinforced four-layer cyanobacterial envelope and is additionally surrounded by two host-derived layers that remodel across the day-night cycle. During daytime N2 fixation, these host-derived barriers become locally discontinuous and the organelle interface becomes enriched with two distinct vesicle populations. Our findings suggest that dynamic control of organelle accessibility through transient membrane gating represents a fundamental strategy by which eukaryotic cells could domesticate new endosymbiotic functions during early organellogenesis.

cell biology↗

bioRxiv2025

Separation of halide oxidation and substrate halogenation chemistries rationalizes site-selective vanadium dependent haloperoxidase catalysis

Vanadium-dependent haloperoxidases (VHPOs) catalyze the halogenation of organic molecules under mild aqueous conditions. Selective bacterial VHPOs exhibit exquisite regio- and enantiocontrol, however the precise mechanisms dictating selectivity have remained elusive. We have solved the single-particle cryo-electron microscopy (cryo-EM) structure of a selective bromoperoxidase from Enhygromyxa salina (esVHPO). Mutagenesis demonstrates that halide oxidation and substrate halogenation occur in two distinct pockets, with halide transfer mediated by critical lysine residue K329. Isolation of a stable intermediate following bromide oxidation (BrOx) enables single turnover catalysis in the presence of organic substrate; subsequent application of a chemoselective fluorescent probe provides support for an intermediate bromamine involved in selectivity. Cryo-EM of the BrOx state reveals a camera shutter mechanism that compacts the halide entry tunnel and vanadate pocket, minimizing the premature dissociation of hypohalous acid. These findings collectively unveil a multilayered halogen trapping and transfer mechanism and provide a rationale for selective VHPO catalysis. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=114 SRC="FIGDIR/small/684477v1_ufig1.gif" ALT="Figure 1"> View larger version (25K): org.highwire.dtl.DTLVardef@6154b9org.highwire.dtl.DTLVardef@169fdorg.highwire.dtl.DTLVardef@d56940org.highwire.dtl.DTLVardef@f11028_HPS_FORMAT_FIGEXP M_FIG C_FIG

biochemistry↗

bioRxiv2025

Structural mechanism for recognition of E2F1 by the ubiquitin ligase adaptor Cyclin F

Cyclin F, a non-canonical member of the cyclin protein family, plays a critical role in regulating the precise transitions of cell-cycle events. Unlike canonical cyclins, which bind and activate cyclin-dependent kinases (CDKs), Cyclin F functions as a substrate receptor protein within the Skp1-Cullin-F box (SCF) E3 ubiquitin ligase complex, enabling the ubiquitylation of target proteins. The structural features that distinguish Cyclin F as a ligase adaptor and the mechanisms underlying its selective substrate recruitment over Cyclin A, which functions in complex with CDK2 at a similar time in the cell cycle, remain largely unexplored. We utilized single-particle cryo-electron microscopy to elucidate the structure of a Cyclin F-Skp1 complex bound to an E2F1 peptide. The structure and biochemical analysis reveal important differences in the substrate-binding site of Cyclin F compared to Cyclin A. Our findings expand on the canonical cyclin-binding motif (Cy or RxL) and highlight the importance of electrostatics at the E2F1 binding interface, which varies for Cyclin F and Cyclin A. Our results advance our understanding of E2F1 regulation and may inform the development of inhibitors targeting Cyclin F.

biochemistry↗

bioRxiv2024

Discovery of three novel neutralizing antibody epitopes on the human astrovirus capsid spike and mechanistic insights into virus neutralization

Human astroviruses (HAstVs) are a leading cause of viral childhood diarrhea that infect nearly every individual during their lifetime. Although human astroviruses are highly prevalent, no approved vaccine currently exists. Antibody responses appear to play an important role in protection from HAstV infection, however knowledge about the neutralizing epitope landscape is lacking, as only 3 neutralizing antibody epitopes have previously been determined. Here, we structurally define the epitopes of 3 uncharacterized HAstV-neutralizing monoclonal antibodies: antibody 4B6 with X-ray crystallography to 2.67 [A], and antibodies 3H4 and 3B4 simultaneously with single-particle cryogenic-electron microscopy to 3.33 [A]. We assess the epitope locations relative to conserved regions on the capsid spike and find that while antibodies 4B6 and 3B4 target the upper variable loop regions of the HAstV spike protein, antibody 3H4 targets a novel region near the base of the spike that is more conserved. Additionally, we found that all 3 antibodies bind with high affinity, and they compete with receptor FcRn binding to the capsid spike. These studies inform which regions of the HAstV capsid can be targeted by monoclonal antibody therapies and could aid in rational vaccine design. ImportanceHuman astroviruses infect nearly every child in the world, causing diarrhea, vomiting, and fever. Despite the prevalence of human astroviruses, little is known about how antibodies block virus infection. Here, we determined high-resolution structures of the astrovirus capsid protein in complex with three virus-neutralizing antibodies. The antibodies bind distinct sites on the capsid spike domain. We find that the antibodies block virus attachment to human cells and prevent capsid spike interaction with the human neonatal Fc receptor. These findings support the use of the human astrovirus capsid spike as an antigen in a vaccine to prevent astrovirus disease.

microbiology↗