Implications of the picornavirus capsid structure for polyprotein processing

Mature picornaviral proteins are derived by progressive, posttranslational cleavage of a precursor polyprotein. These cleavages play a role in the control of virus functions. Although the processed termini are separated by as much as 75 A in the native virus capsid, the fold and arrangement of polypeptide chains in a protomer before proteolysis are likely to be similar to that found in the mature virus. The three-dimensional structures of rhinovirus and Mengo virus suggest that the cleavage sites within the protomeric precursor are in structurally flexible regions. The final proteolytic processing event, maturation of the virion peptide VP0 (also called peptide 1AB) appears to occur by an unusual autocatalytic serine protease-type mechanism possibly involving viral RNA basic groups that would serve as proton-abstractors during the cleavage reaction.

Comparison of the three-dimensional structure of two human rhinoviruses (HRV2 and HRV14)

An attempt has been made to build a model of human rhinovirus 2 (HRV2) based on the known human rhinovirus 14 (HRV14) structure. HRV2 was selected because its amino acid sequence is known and because it belongs to the minor rhinovirus receptor class as compared to HRV14, which belongs to the major class. Initial alignment of HRV2 with HRV14 based on the primary sequence and the knowledge of the three-dimensional structure of HRV14 showed that the most probable position of the majority of insertions and deletions occurred in the vicinity of the neutralizing immunogenic sites (NIm). Out of a total of 855 amino acids present in one copy of each of the capsid proteins VP1 through VP4 of HRV14, 411 are different between the two viruses. There are also 6 amino acid residues inserted and 14 residues deleted in HRV2 relative to HRV14. Examination of amino acid interactions showed several cases of conservation of function, e.g., salt bridges or the filling of restricted space. The largest variation amongst the residues lining the canyon, the putative receptor binding site, was in the carboxy-terminal residues of VP1.

The site of attachment in human rhinovirus 14 for antiviral agents that inhibit uncoating

WIN 51711 and WIN 52084 are structurally related, antiviral compounds that inhibit the replication of rhino (common cold) viruses and related picornaviruses. They prevent the pH-mediated uncoating of the viral RNA. The compounds consist of a 3-methylisoxazole group that inserts itself into the hydrophobic interior of the VP1 beta-barrel, a connecting seven-membered aliphatic chain, and a 4-oxazolinylphenoxy group (OP) that covers the entrance to an ion channel in the floor of the "canyon." Viral disassembly may be inhibited by preventing the collapse of the VP1 hydrophobic pocket or by blocking the flow of ions into the virus interior.

Ontogenesis of nuclear T3 receptors in primary cultured astrocytes and neurons

Nuclear T3 receptor (NTR) have been characterized in separated cultures of neurons and astrocytes. Scatchard analysis indicated the presence of a single class of high-affinity sites in both cell lines. The apparent equilibrium association constant ranged from 1.80 +/- 0.41 X 10(10) M-1 to 3.27 +/- 0.74 +/- 10(10) M-1 in neurons and from 1.01 +/- 0.09 to 1.80 +/- 0.73 X 10(10) M-1 in astrocytes depending on the time in culture. In neurons, the maximal binding capacity (MBC) increased from 0.049 +/- 0.008 ng T3/mg DNA to 0.328 +/- 0.052 ng T3/mg DNA between 3 and 12 days of culture. In astrocytes, the changes in MBC were less pronounced ranging from a minimum of 0.095 +/- 0.024 ng T3/mg DNA at the 7th day of culture to a maximum of 0.198 +/- 0.048 ng T3/mg DNA at the 21st day. The relative binding affinity of the receptor for thyroid hormone analogs was in the order TRIAC greater than L-T3 greater than D-T3 greater than L-T4 in both cell lines. These results show that nuclear T3 receptors similar to those found in vivo are present in primary cultures of both astrocytes and neurons.

Regulation of protein phosphorylation by triiodothyronine (T3) in neural cell cultures. Part I: Astrocytes

Dissociated cells from 2-day-old rat cerebral hemispheres were cultured for 17 days in absence of thyroid hormones using conditions yielding mainly glial cells. Triiodothyronine (10(-8) M) was added for 0-72 h before the end of the incubation and [32P]phosphate was added for the last 4 h. Soluble (105,000 X g supernatant), particulate (105,000 X g pellet) and HMG (high mobility group; 0.75 M perchloric acid-soluble proteins) fractions were prepared and phosphorylated proteins in each fraction were analyzed by polyacrylamide gel electrophoresis. In the soluble fraction a protein (Mr = 19,000) incorporates less [32P]phosphate after only 4 h of T3 treatment. The maximal effect is attained after 7 h (-42%) and remains unchanged up until 72 h. In this fraction, the phosphorylation of some other proteins is increased but the maximal effect is observed 48 and 72 h after T3 administration. In the particulate fraction, exposure to T3 rapidly (4 h) increases the amount of a protein (Mr = 45,000) identified as beta-actin. Protein phosphorylation in this fraction is slightly, or not at all, affected by T3. In contrast, a rapid (between 4 and 7 h) increased phosphorylation of a 17 kDa protein in the HMG fraction is observed following T3 stimulation. This nuclear protein was further characterized as HMG 14. These results show that thyroid hormones can produce direct effects (not mediated by neurons) on the phosphorylation of specific proteins in cultured glial cells. Possible functional implications of the observed protein changes are discussed in this paper.

Regulation of protein phosphorylation by triiodothyronine (T3) in neural cell cultures. Part II: Neurons

Cerebral hemisphere from 16- to 18-day-old rat fetuses were dissociated and cells were cultured in absence of thyroid hormones. Neuron-enriched cultures were obtained either by using cells after 6 days of culture (before extensive glial cell proliferation) or by adding cytosine arabinoside for 48 h after 4 days of culture and using cells on day 9. Cells were incubated with T3 (10(-8) M) for 0-72 h and [32P]phosphate was added for the last 4 h of incubation. HMG (high mobility group; 0.75 M perchloric acid-soluble proteins) were prepared and phosphorylated proteins were analyzed by polyacrylamide gel electrophoresis. T3 rapidly (4-7 h) increased the phosphorylation of histone H1 and of a protein with apparent molecular mass of 17000 Da identified as HMG 14. In addition, in cells not treated with cytosine arabinoside, histone H1 was resolved into 3 subfractions and each of these responded to the hormone with a different time course. These results indicate that thyroid hormones act on the phosphorylation of specific nuclear proteins and therefore may influence chromatin structure and gene expression in primary neuronal cell cultures.

Picornaviruses of two different genera have similar structures

Crystals of Mengo virus were used to collect three-dimensional X-ray diffraction data to 7 A resolution. A self-rotation function showed the precise orientation of the Mengo particles in the crystal unit cell. A cross-rotation function against similar data of cubic rhinovirus crystals showed a peak when the orientations of these two icosahedral viruses were superimposed. This demonstrates similarity of capsid construction between two picornaviruses of different taxonomic genera.

Virion orientation in cubic crystals of the human common cold virus HRV14

A new cubic crystal form (a = 445.1 A) of space group P23 is reported for human rhinovirus R14. There are four particles per unit cell, each situated on a crystallographic 3-fold axis. The orientation of these particles has been determined with a rotation function and their approximate positions have been derived from a Patterson map. The crystals diffract to at least 2.8 A resolution. Limitations to the possible surface features of the virus are set by a comparison of the cubic and orthorhombic crystal forms.