Human parainfluenza virus type 3 HN-receptor interaction: Effect of 4-guanidino-neu5Ac2en on a neuraminidase-deficient variant

The envelope of human parainfluenza virus type 3 (HPF3) contains two viral glycoproteins, the hemagglutinin-neuraminidase (HN) and the fusion protein (F). HN, which is responsible for receptor attachment and for promoting F-m ediated fusion, also possesses neuraminidase (receptor-destroying) activity . We reported previously that 4-guanidino-neu5Ac2en (4-GU-DANA) and related sialic acid-based inhibitors of HPF3 neuraminidase activity also inhibit H N-mediated receptor binding and fusion processes not involving neuraminidas e activity. We have now examined this mechanism, as well as neuraminidase's role in the viral life cycle, using a neuraminidase-deficient HPF3 variant (C28a) and stable cell lines expressing C28a or wild-type (wt) HN. C28a, w hich has a wt F sequence and two point mutations in the HN gene correspondi ng to two amino acid changes in the HN protein, is the first HPF3 variant w ith insignificant neuraminidase activity. Cells expressing C28a RN did not bind erythrocytes at 4 degreesC unless pretreated with neuraminidase, but n o such pretreatment was required for hemadsorption activity (HAD) at 22 or 37 degreesC. HAD was blocked by 4-GU-DANA, attesting to the ability of this compound to inhibit HN's receptor-binding activity. C28a or wt plaque enla rgement, a process that involves cell-cell fusion and does not depend on vi rion release, is diminished by the presence of 4-GU-DANA, confirming the in hibitory effect of 4-GU-DANA on the fusogenic function of C28a HN. In C28a- infected cell monolayers, virion release and thus multicycle replication ar e severely restricted. This defect was corrected by supplementation of exog enous neuraminidase and also by the addition of 4-GU-DANA; neuraminidase de stroys the receptors whereby newly formed C28a virions would remain attache d to the cell surface, whereas 4-GU-DANA prevents the attachment itself, ob viating the need for receptor cleavage. In accord with the ability of 4-GU- DANA to prevent attachment, the neuraminidase inhibitory effect of 4-GU-DAN A on wt HPF3 did not diminish virion release into the medium. Thus, it is b y inhibition of viral entry and syncytium formation that sialic acid analog s like 4-GU-DANA may counteract wt HPF3 infection.