1.A.68 The Viral Small Hydrophobic Viroporin (V-SH) Family
The small hydrophobic (SH) protein from the human respiratory syncytial virus (hRSV) is a glycoprotein of 64 amino acids with one putative TMS and channel activity. Although SH proteins are important for viral infectivity, their exact roles during viral infection are not clear. Gan et al., 2008 studied the secondary structure, orientation, and oligomerization of the transmembrane domain of SH (SH-TM) in the presence of lipid bilayers. Only one oligomer, a pentamer, was observed.
Conductance studies of SH-TM demonstrated ion channel activity which is cation selective and inactive below the predicted pK(a) of histidine. Thus, the transmembrane domain of the SH protein forms pentameric alpha-helical bundles that form cation-selective ion channels in planar lipid bilayers. Gan et al., 2008 suggest a model for this pore.
The absence of the small hydrophobic (SH) protein encoded by the human respiratory syncytial virus (hRSV) leads to viral attenuation and prevents apoptosis in infected cells. Gan et al. (2012) examined the structure of the SH protein in detergent micelles and in lipid bilayers. In detergent micelles, the TM domain is flanked N-terminally by a α-helix that forms a ring around the lumen of the pore, and C-terminally by an extended β-turn. The SH protein was found in the plasma membrane of transiently expressing HEK293 cells, which showed pH-dependent (acid-activated) channel activity. Channel activity was abolished in mutants lacking both native His residues, H22 and H51, but not when either His was present. Gan et al. (2012) proposed that the pentameric SH protein is a physiologically relevant conformation, albeit probably not the only one, in which SH contributes to RSV infection and replication.
The generalized reaction catalyzed by V-SHP is:
ions (in) ⇌ ions (out)
References:
The viral small hydrophobic protein (V-SHP; hRSV-SH) of 64 aas with 1 TMS, It forms a pentameric ion conducting pore in the membrane (Surya and Torres 2015) that transports monovalent cations (Hyser and Estes 2015). The SH protein has two protonatable His residues in its transmembrane domain that are oriented facing the lumen of the channel. Their protonation may serve as a pH sensor, to promote electrostatic repulsion and reduced oligomer stability at low pH (Surya and Torres 2015). Pyronin B can reduce SH channel activity, and its likely binding site on the SH protein channel has been identified. Black lipid membrane experiments confirmed that protonation of both histidine residues reduces stability and channel activity (Li et al. 2014). Water transport was observed with histidine residues of five chains (His22 and His51) playing a key role in pore permeability (Araujo et al. 2016). The assembly of the transmembrane domains of viral channel-forming proteins and peptide drug screening has been achieved using a docking approach (Huang and Fischer 2022).
Animal virus
SH protein of human respiratory syncytial virus (P04852)
BSV small hydrophobic (SH) protein of 81 aas (Karger et al. 2001).
Viruses
SH of bovine respiratory syncytial virus
Small hydrophobic viroporin protein (SH), also called small protein 1A, of 65 aas and 1 TMS. Forms a proton-selective cation channel that may also be capable of transporting Na+ and K+, playing a role in budding and /or virus entry (Scott and Griffin 2015). May also play a role in counteracting host innate immunity (Russell et al. 2015). The SH protein is stable in its pentameric membrane-integrated form. Simulations also showed the presence of water molecules within the bilayer by density distribution, thus confirming that the SH protein is a viroporin (Araujo et al. 2016).
Viroporin SH of human respiratory syncytial virus B