Hydrophobic and Charged Residues in the Central Segment of the Measles Virus Hemagglutinin Stalk Mediate Transmission of the Fusion-Triggering Signal

Abstract

The pH-independent measles virus membrane fusion process begins when the attachment protein H binds to a receptor. Knowing that the central segment of the tetrameric H stalk transmits the signal to the fusion protein trimer, we investigated how. We document that exact conservation of most residues in the 92 through 99 segment is essential for function. In addition, hydrophobic and charged residues in the 104 through 125 segment, arranged with helical periodicity, are critical for F protein interactions and signal transmission.

Fusion Activation through Attachment Protein Stalk Domains Indicates a Conserved Core Mechanism of Paramyxovirus Entry into Cells

“…The similarity of NDV and PIV5 HN stalk F-activating regions are such that maintaining the relative positions of critical hydrophobic residues in the context of the 4HB is sufficient to activate NDV F by a chimeric NDV HN protein containing part of the PIV5 HN F-activating region. In addition, we show a requirement for stalk flexibility at the F activation region of NDV HN similar to that observed for MeV and CDV H proteins (43,44,56). Lastly, we show that like PIV5 HN, MeV H, and NiV G, "headless" stalk domains of MuV HN and NDV HN, can trigger F and cause cellcell fusion.…”

“…Furthermore, introduction of disulfide bonds in the upper region of the MeV H stalk did not cause a major functional disruption (56). Thus, neither the introduction of rigid disulfide bridges nor the addition of bulky carbohydrate moieties in the 4HB of the upper stalk appeared to disrupt any putative conformational changes that may propagate down through this upper stalk region.…”

“…For most members of the Paramyxoviridae family the attachment protein stalk domain is believed to interact with and trigger the metastable F protein to initiate fusion. Recent studies on PIV5 HN, NDV-HN, MeV H, and CDV H have implicated the F-interacting and F-activating regions to be in the central part of the HN or H stalk (29,33,41,43,44,56,60). Recently, we identified residues located within and near the PIV5-F "Iglike" domain as putative HN-interacting residues (61).…”

“…A requirement of some degree of conformational flexibility in the central part of the 4HB stalk of morbilliviruses (MeV and CDV) (43,44,56), as well as energy-dependent conformational changes possibly occurring between globular head dimers (42,55), has been implicated as critical for the correct timing of F activation on receptor binding. We demonstrate here that a residue within the NDV HN F activation domain (S92) that does not directly affect fusion activation when mutated singly (33) is able to form disulfide-linked dimers and in the process blocks the activation of both NDV F and a hyperfusogenic mutant of NDV F (L289A).…”

Measles Virus Fusion Machinery Activated by Sialic Acid Binding Globular Domain

“…In this report, chimeric H-HN envelope glycoproteins (15,16,42,(46)(47)(48) were used to evaluate the interplay between the globular head and the stalk region that leads to fusion protein activation and to determine whether the transmission of the fusion signal from head to stalk is similar whether the virus binds to sialic acid or proteinaceous receptors.…”