Rosita Moser

The cellular receptor to human rhinovirus 2 binds around the 5-fold axis and not in the canyon: a structural view

Human rhinovirus serotype 2 (HRV2) belongs to the minor group of HRVs that bind to members of the LDL‐receptor family including the very low density lipoprotein (VLDL)‐receptor (VLDL‐R). We have determined the structures of the complex between HRV2 and soluble fragments of the VLDL‐R to 15 Å resolution by cryo‐electron microscopy. The receptor fragments, which include the first three ligand‐binding repeats of the VLDL‐R (V1–3), bind to the small star‐shaped dome on the icosahedral 5‐fold axis. This is in sharp contrast to the major group of HRVs where the receptor site for ICAM‐1 is located at the base of a depression around each 5‐fold axis. Homology models of the three domains of V1–3 were used to explore the virus–receptor interaction. The footprint of VLDL‐R on the viral surface covers the BC‐ and HI‐loops on VP1.

Dynamic force microscopy imaging of native membranes

We employed magnetic ACmode atomic force microscopy (MACmode AFM) as a novel dynamic force microscopy method to image surfaces of biological membranes in their native environments. The lateral resolution achieved under optimized imaging conditions was in the nanometer range, even when the sample was only weakly attached to the support. Purple membranes (PM) from Halobacterium salinarum were used as a test standard for topographical imaging. The hexagonal arrangement of the bacteriorhodopsin trimers on the cytoplasmic side of PM was resolved with 1.5nm lateral accuracy, a resolution similar to images obtained in contact and tapping-mode AFM. Human rhinovirus 2 (HRV2) particles were attached to mica surfaces via nonspecific interactions. The capsid structure and 2nm sized protein loops of HRV2 were routinely obtained without any displacement of the virus. Globular and filamentous structures on living and fixed endothelial cells were observed with a resolution of 5-20nm. These examples show that MACmode AFM is a favorable method in studying the topography of soft and weakly attached biological samples with high resolution under physiological conditions.

A Cellular Receptor of Human Rhinovirus Type 2, the Very-Low-Density Lipoprotein Receptor, Binds to Two Neighboring Proteins of the Viral Capsid

ABSTRACT The very-low-density lipoprotein receptor (VLDL-R) is a receptor for the minor-group human rhinoviruses (HRVs). Only two of the eight binding repeats of the VLDL-R bind to HRV2, and their footprints describe an annulus on the dome at each fivefold axis. By studying the complex formed between a selection of soluble fragments of the VLDL-R and HRV2, we demonstrate that it is the second and third repeats that bind. We also show that artificial concatemers of the same repeat can bind to HRV2 with the same footprint as that for the native receptor. In a 16-Å-resolution cryoelectron microscopy map of HRV2 in complex with the VLDL-R, the individual repeats are defined. The third repeat is strongly bound to charged and polar residues of the HI and BC loops of viral protein 1 (VP1), while the second repeat is more weakly bound to the neighboring VP1. The footprint of the strongly bound third repeat extends down the north side of the canyon. Since the receptor molecule can bind to two adjacent copies of VP1, we suggest that the bound receptor “staples” the VP1s together and must be detached before release of the RNA can occur. When the receptor is bound to neighboring sites on HRV2, steric hindrance prevents binding of the second repeat.

Viral Evolution toward Change in Receptor Usage: Adaptation of a Major Group Human Rhinovirus To Grow in ICAM-1-Negative Cells

ABSTRACT Major receptor group common cold virus HRV89 was adapted to grow in HEp-2 cells, which are permissive for minor group human rhinoviruses (HRVs) but which only marginally support growth of major-group viruses. After 32 blind passages in these cells, each alternating with boosts of the recovered virus in HeLa cells, HRV89 acquired the capacity to effectively replicate in HEp-2 cells, attaining virus titers comparable to those in HeLa cells although no cytopathic effect was observed. Several clones were isolated and shown to replicate in HeLa cells whose ICAM-1 was blocked with monoclonal antibody R6.5 and in COS-7 cells, which are devoid of ICAM-1. Blocking experiments with recombinant very-low-density lipoprotein receptor fragments and enzyme-linked immunosorbent assays indicated that the mutants bound a receptor different from that used by minor-group viruses. Determination of the genomic RNA sequence encoding the capsid protein region revealed no changes in amino acid residues at positions equivalent to those involved in the interaction of HRV14 or HRV16 with ICAM-1. One mutation was within the footprint of a very-low-density lipoprotein receptor fragment bound to minor-group virus HRV2. Since ICAM-1 not only functions as a vehicle for cell entry but has also a “catalytic” function in uncoating, the use of other receptors must have important consequences for the entry pathway and demonstrates the plasticity of these viruses.

Monitoring RNA Release from Human Rhinovirus by Dynamic Force Microscopy

ABSTRACT Human rhinoviruses were imaged under physiological conditions by dynamic force microscopy. Topographical images revealed various polygonal areas on the surfaces of the 30-nm viral particles. RNA release was initiated by exposure to a low-pH buffer. The lengths of the RNAs that were released but still connected to the virus capsid varied between 40 and 330 nm, whereas RNA molecules that were completely released from the virus were observed with lengths up to 1 μm. Fork-like structure elements with 30-nm extensions were sometimes resolved at one end of the RNA molecules. They possibly correspond to the characteristic multi-stem-loop conformation, the internal ribosomal entry site, located at the 5′ region of the genome. This study demonstrates that dynamic force microscopy can be used to study viral RNA release in situ under physiological conditions.

Quasi‐crystalline Arrangement of Human Rhinovirus 2 on Model Cell Membranes

We used dynamic force microscopy to study the attachment of human rhinovirus 2 (HRV2) particles to model membranes. A supported lipid bilayer containing Ni2+-nitrilotriacetate (NTA)-lipids as functional groups in the outer leaflet at high surface density was assembled on mica and showed a flat extended structure of 3 nm in height. The recombinant very-low density lipoprotein receptor fragment MBP-VLDLR1-8, consisting of the ligand binding domain (all eight complement type A repeats) fused at the N-terminus to maltose binding protein (MBP) and containing a hexahistidine (His6)-tag at its C-terminus, was attached to the membranes via the His6-Ni2+NTA bond and revealed monolayer density. The protein-lipid layer was then used as a model to investigate virus binding to its receptor in a planar membrane. The viral particles attached in a tightly packed, long range order quasi-crystalline arrangement. This assembly is thus ideally suited for studies of viral dynamics and recognition under physiological conditions.

Dynamic force microscopy for imaging of viruses under physiological conditions.

Dynamic force microscopy (DFM) allows imaging of the structure and the assessment of the function of biological specimens in their physiological environment. In DFM, the cantilever is oscillated at a given frequency and touches the sample only at the end of its downward movement. Accordingly, the problem of lateral forces displacing or even destroying bio-molecules is virtually inexistent as the contact time and friction forces are reduced. Here, we describe the use of DFM in studies of human rhinovirus serotype 2 (HRV2) weakly adhering to mica surfaces. The capsid of HRV2 was reproducibly imaged without any displacement of the virus. Release of the genomic RNA from the virions was initiated by exposure to low pH buffer and snapshots of the extrusion process were obtained. In the following, the technical details of previous DFM investigations of HRV2 are summarized.