The H6N1 influenza A virus is an enveloped RNA virus with potential for zoonosis (transmission from animals to humans).  It obtains its plasma membrane from its host cell, but the composition of the viral envelope and vertebrate plasma membrane are distinct.  The viral envelope consists mostly of phosphatidylethanolamine as well as some negatively charged glycerophospholipids.  The outer membrane contains hemagglutinin (HA) proteins that bind to the carbohydrate known as sialic acid (SA) on host cell receptor proteins, facilitating viral entry.  It also contains neuraminidase enzymes that break the bond between SA and galactose in the membrane glycans of various host species, allowing new virions to be released.Minute changes in the genes that code for HA proteins can alter the virulence, host, and attachment mechanism.  In general, influenza A infects avian species by attaching to tracheal tissue, which displays SA groups bound to receptors through an α-2,3-glycosidic linkage to galactose (Figure 1).  In contrast, human cells contain α-2,6-glycosidic linkages between SA and galactose; avian viruses generally cannot attack human cells due to poor recognition of these linkages by H6N1 HA proteins.Figure 1  Structure of the influenza A receptor in avian cell membranesIn a recent study, however, a G225D substitution in HA, caused by a GGC to GAU codon mutation, enabled avian virions to infect human cells.  These virions are known as the H6-G225D strain.  The stability of G225D HA proteins was compared to that of the HA found in the H6-P186L strain, which is created by a single CCC to CUC mutation and exclusively infects avian cells, and to the stability of HA isolated from H1N1 virions, which targets only humans.  The viral RNA of H6-P186L, H6-G225D, and H1N1 was cloned using cDNA and expressed in embryonic kidney cells.  The melting curve of each purified HA protein was determined using differential scanning calorimetry, with peaks representing melting temperatures (Figure 2).Figure 2  Hemagglutinin melting curves for H6N1 and H1N1 influenza A strainsAdapted from de Vries RP, Tzarum N, Peng W, et al. A single mutation in Taiwanese H6N1 influenza hemagglutinin switches binding to human-type receptors. EMBO Mol Med. 2017;9(9):1314-1325. doi:10.15252/emmm.201707726 Question 22On human receptors, neuraminidase action would lead to a decrease in what type of bond?A.N-linked glycosidic bondB.β-1,4-glycosidic bondC.α-2,3-glycosidic bondD.α-2,6-glycosidic bond