Papillomaviruses propagate in differentiating skin cells, and certain types are responsible for the onset of cervical cancer. We have combined image reconstructions from electron cryomicroscopy (cryoEM) of bovine papillomavirus at 9 A Ê resolution with coordinates from the crystal structure of small virus-like particles of the human papillomavirus type 16 L1 protein to generate an atomic model of the virion. The overall ®t of the L1 model into the cryoEM map is excellent, but residues 402±446 in the`C-terminal arm' must be rebuilt. We propose a detailed model for the structure of this arm, based on two constraints: the presence of an intermolecular disul®de bond linking residues 175 and 428, and the clear identi®cation of a feature in the image reconstruction corresponding to an a-helix near the C-terminus of L1. We have con®rmed the presence of the disul®de bond by mass spectrometry. Our`invading arm' model shows that papilloma-and polyomaviruses have a conserved capsid architecture. Most of the rebuilt C-terminal arm is exposed on the viral surface; it is likely to have a role in infection and in immunogenicity. Keywords: disul®de bond/electron microscopy/ papillomavirus/vaccine design/virus assembly IntroductionPapillomaviruses are double-stranded DNA viruses that propagate in differentiating skin cells. Over 100 human types been identi®ed, of which a fewÐnotably types 16, 18, 31 and 45Ðare known to be responsible for the onset of cervical cancer (Bosch et al., 1995;Parkin et al., 2000), the second most common cancer among women worldwide (Parkin et al., 2000). Papilloma virions are nonenveloped particles. Their outer shell,~600 A Ê in diameter, contains 72 pentamers of the protein L1, situated at the vertices of a T = 7dextro icosahedral lattice ( Figure 1B; Baker et al., 1991). Cellular histones condense the 8 kb genome into a double-stranded, covalently closed circular DNA minichromosome (Howley, 1996).Electron microscopy (EM) of virions in vitreous ice reveals that cottontail rabbit papillomavirus, bovine papillomavirus type 1 (BPV1) and human papillomavirus type 1 (HPV1) have essentially identical structures (Baker et al., 1991;Hagensee et al., 1994;Belnap et al., 1996;Trus et al., 1997); indeed, their L1 subunits have nearly 50% sequence identity. The atomic structure of recombinant HPV16 L1 has been determined from crystals of a 12-pentamer assembly, or small virus-like particle (small VLP), stable at low pH ( Figure 1C±F; Chen et al., 2000). Despite a lack of signi®cant sequence similarity, the architecture of the papillomavirus L1 pentamer closely resembles that of the VP1 pentamers of simian virus 40 (SV40) and murine polyomavirus: a ring of ®ve b-jellyroll domains, tightly linked by interacting loops between framework b-strands (Figure 1E and F;Liddington et al., 1991;Stehle et al., 1994). A C-terminal subdomain of the subunit mediates pentamer±pentamer contacts in the small VLPs of HPV16. The polypeptide chain exits the b-jellyroll, forms the protruding subdomain, and reinserts into the central ch...

The HUSH complex represses retroviruses, transposons and genes to maintain the integrity of vertebrate genomes. HUSH regulates deposition of the epigenetic mark H3K9me3, but how its three core subunits — TASOR, MPP8 and Periphilin — contribute to assembly and targeting of the complex remains unknown. Here, we define the biochemical basis of HUSH assembly and find that its modular architecture resembles the yeast RNA-induced transcriptional silencing complex. TASOR, the central HUSH subunit, associates with RNA processing components. TASOR is required for H3K9me3 deposition over LINE-1 repeats and repetitive exons in transcribed genes. In the context of previous studies, this suggests that an RNA intermediate is important for HUSH activity. We dissect the TASOR and MPP8 domains necessary for transgene repression. Structure-function analyses reveal TASOR bears a catalytically-inactive PARP domain necessary for targeted H3K9me3 deposition. We conclude that TASOR is a multifunctional pseudo-PARP that directs HUSH assembly and epigenetic regulation of repetitive genomic targets.

The COVID-19 disease is one of worst pandemics to sweep the globe in recent times. It is noteworthy that the disease has its greatest impact on the elderly. Herein, we investigated the potential of childhood vaccination, specifically against measles, mumps and rubella (MMR), to identify if this could potentially confer acquired protection over SARS-CoV-2. We identified sequence homology between the fusion proteins of SARS-CoV-2 and measles and mumps viruses. Moreover, we also identified a 29% amino acid sequence homology between the Macro (ADP-ribose-1''-phosphatase) domains of SARS-CoV-2 and rubella virus. The rubella Macro domain has surface-exposed conserved residues and is present in the attenuated rubella virus in MMR. Hence, we hypothesize that MMR could protect against poor outcome in COVID-19 infection. As an initial test of this hypothesis, we identified that 1) age groups that most likely lack of MMR vaccine-induced immunity had the poorest outcome in COVID-19, and 2) COVID-19 disease burden correlates with rubella antibody titres, potentially induced by SARS-CoV2 homologous sequences. We therefore propose that vaccination of 'at risk' age groups with an MMR vaccination merits further consideration as a time appropriate and safe intervention.