Mechanisms of entry by single-stranded RNA viruses
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Respiratory Syncytial Virus (RSV) is a single-stranded RNA virus that belongs to the Pneumovirus genus of the Pneumovirinae subfamily, which is part of the Paramyxoviridae family. It is the leading cause of lower respiratory tract disease in infants and young children, consisting of severe bronchiolitis and pneumonia (Glezen & Denny, 1973; Holberg et al., 1991; Krilov, 2001). Unfortunately, even after decades of research, no effective treatment is available, and prospects of a vaccine are meager so far. Currently, it has been established that RSV G protein binds to GAGs, specifically HS and CS-B, but also that the F protein on its own is able to determine the specificity of entry with G protein only enhancing the efficiency of F-mediated entry. Even though Hallak et al. (2000) suggest that HS or CS-B is a specific receptor for RSV, there is no definite receptor known yet beyond these speculations. However, in order to progress to further speculations on potential RSV receptors, it might be useful to look at receptors utilized by other single-stranded RNA viruses. Some of the Paramyxoviridae, such as hMPV, have also been implicated in GAG-binding. However, it is unlikely that cellular GAGs are the main determinants of species-specific infection by these viruses, and more species or cell specific receptors must be implicated in infection as well. So far, specific receptors have only been identified for several Paramyxoviridae, such as the Nipah Virus (ephrinB2), Measles Virus (SLAM/CD46) and Sendai Virus (specific gangliosides, such as ASGP-R). Even though some of the techniques used to identify these receptors might be useful to researchers trying to identify potential RSV receptors, the receptors utilized by the aforementioned single-stranded RNA viruses are unfortunately unrelated and therefore no distinct pattern has emerged from the information obtained. Insight into mechanisms of entry by other single-stranded RNA viruses has therefore not provided further clues on potential receptors for RSV.