The footprint of the antibody covers loop 3 of the CPV coat protein VP2, which is the site of all the known escape mutations for this antibody, and also loops 1 and 2. Fab molecule is oriented in a near radial direction, inclined away from the two-fold axes. The viral epitope consists of 14 amino acid residues found in loops 1, 2 and 3 on the capsid surface, which include previously identified escape mutations. == Conclusions == The mode of Fab binding suggests that the A3B10 neutralizing antibody cannot bind bivalently to the capsid across the two-fold axes, consistent with the observation that whole A3B10 antibody readily precipitates CPV. Since Fab A3B10 can also neutralize the virus, mechanisms of neutralization such as interference with cell attachment, cell entry, or uncoating, must Darunavir Ethanolate (Prezista) be operative. Keywords:antibodyvirus complex, antigenic surface, cryo-electron microscopy, neutralization of virus, parvovirus == Introduction == Members of theParvovirusgenus cause a variety of diseases in mammals, including humans, particularly in pregnant females and in newborns. Diseases include enteritis, in the case of canine parvovirus (CPV) [1,2], and childhood fifth disease, caused by the human pathogen B19 [3,4]. Parvoviruses infect only actively proliferating Darunavir Ethanolate (Prezista) (S phase) cells [5]. They have a diameter of approximately 255, a molecular mass between 5.0 106and 6.2 106daltons, contain a single-stranded DNA genome of about 5000 bases and have aT= 1 icosahedral capsid. The structural motif of each of the 60 capsid subunits is an eight-stranded antiparallel -barrel, common to most icosahedral viruses [6]. CPV is most closely related to feline panleukopenia virus (FPV), raccoon parvovirus and mink enteritis virus. The major capsid component is virus protein 2 (VP2), with a small amount of VP3. VP3 results from a proteolytic cleavage of approximately 17 residues from the amino terminus of VP2 [7,8]. In CPV, VP1 contains 143 additional Darunavir Ethanolate (Prezista) amino-terminal residues compared with VP2. The three-dimensional structures of CPV and FPV have been determined to atomic resolution [6,9] and of human B19 to 8 resolution (M Agbandje, S Kajigaya, R McKenna, NS Young & MG Rossmann, unpublished data), using X-ray crystallography. The immune system in vertebrates is the primary mechanism by which infectious viruses are detected and neutralized. Several hypotheses have been proposed to explain how neutralization occurs [10]. For example, antibodies might bind monovalently to a virus and thereby achieve interparticle cross-linking and precipitation of the infectious particles. Alternatively, some neutralizing antibodies bind bivalently. In the case of human rhinovirus 14 (HRV14), the neutralizing monoclonal antibody (Mab) Mab17-1A binds across the icosahedral two-fold axes, thus maintaining the twofold symmetry of the Darunavir Ethanolate (Prezista) antibody [10,11]. Thus, in this instance, neutralization is not accompanied by precipitation of the virus; the mechanism of neutralization might be inhibition of receptor recruitment, of uncoating, or of other factors. Antibody fragment (Fab) Fab17 1A bridges the HRV14 canyon, which is the site of receptor binding, and thus might neutralize the virus by blocking a sufficient number of receptor attachment sites [10]. Neutralizing antigenic sites have been determined on the CPV surface, using escape mutant analysis and peptide mapping [1215] (Table 1,Fig. 1). Two epitopes (A and B) have been identified by analysis of mutations which inhibit antibody neutralization. Site A is on the three-fold spike, involving loops 1 and 2. Site B is on a ridge between the canyon and the depression at the two-fold axis. Epitopes corresponding roughly to sites A and B have also been identified by pepscan mapping [1214]. Additional Mouse monoclonal to GST antigenic sites have been identified by peptide mapping. Epitopes in the amino terminus of VP1, corresponding to residue positions 2 to +21 in VP2, bind antibodies that neutralize CPV. These residues may protrude through the five-fold axes [6]. == Table 1. == CPV epitopes. == Fig. 1. == (opposite). Roadmap showing the surface amino acids of CPV for one asymmetric unit of the icosahedral particle. The radial distance of the surface from the viral center is color-coded at the top. Boundaries between symmetry-related polypeptides are indicated with a heavy black line. The lower figure shows (shaded) epitopes identified by pepscan or peptide mapping. Epitopes on the amino terminus of VP1 identified by pepscan and not present in the atomic structure are indicated as a line from the five-fold axis. Escape mutations to Mab A3B10 are indicated in red (epitope A) and blue (epitope B). (The figure was produced in part by the program Roadmap [46]). Here we.
