Structural analysis of the roles of influenza A virus membrane-associated proteins in assembly and morphology

The assembly of influenza A virus at the plasma membrane of infected cells leads to release of enveloped virions that are typically round in tissue culture-adapted strains but filamentous in strains isolated from patients. The viral proteins hemagglutinin (HA), neuraminidase (NA), matrix protein 1 (M1), and M2 ion channel all contribute to virus assembly. When expressed individually or in combination in cells, they can all, under certain conditions, mediate release of membrane-enveloped particles, but their relative roles in virus assembly, release, and morphology remain unclear. To investigate these roles, we produced membrane-enveloped particles by plasmid-derived expression of combinations of HA, NA, and M proteins (M1 and M2) or by infection with influenza A virus. We monitored particle release, particle morphology, and plasma membrane morphology by using biochemical methods, electron microscopy, electron tomography, and cryo-electron tomography. Our data suggest that HA, NA, or HANA (HA plus NA) expression leads to particle release through nonspecific induction of membrane curvature. In contrast, coexpression with the M proteins clusters the glycoproteins into filamentous membrane protrusions, which can be released as particles by formation of a constricted neck at the base. HA and NA are preferentially distributed to differently curved membranes within these particles. Both the budding intermediates and the released particles are morphologically similar to those produced during infection with influenza A virus. Together, our data provide new insights into influenza virus assembly and show that the M segment together with either of the glycoproteins is the minimal requirement to assemble and release membrane-enveloped particles that are truly virus-like.

Corporate board and corporate social responsibility assurance: Evidence from China

This paper investigates the association between board characteristics and the company’s corporate social responsibility (CSR) assurance decision in China. By examining 2054 firm-years of Chinese listed companies with CSR reports from 2008 to 2012, we find that firms with a large board size, more female directors, and separation of CEO and chairman positions are more likely to engage in CSR assurance. Gender diversity also influences the CSR assurance provider choice. However, board independence and overseas background of the CEO do not affect the CSR assurance decision. Inconsistent with our prediction, firms with foreign directors are less likely to engage in voluntary CSR assurance. In summary, this research provides in-depth insights into the determinants of Chinese firms’ voluntary CSR assurance.