Organization of capsid-associated tegument components in Kaposi's sarcoma-associated herpesvirus

Abstract

© 2014, American Society for Microbiology. Capsid-associated tegument proteins have been identified in alpha- and betaherpesviruses to play an essential role in viral DNA packaging. Whether and how such tegument proteins exist in gammaherpesviruses have been mysteries. Here, we report a 6-Åresolution cryo-electron microscopy (cryo-EM) structure of Kaposi's sarcoma-associated herpesvirus (KSHV) virion, a member of the oncogenic gammaherpesvirus subfamily. The KSHV virion structure reveals, for the first time, how capsid-associated tegument proteins are organized in a gammaherpesvirus, with five tegument densities capping each penton vertex, a pattern highly similar to that in alphaherpesvirus but completely different from that in betaherpesvirus. Each KSHV tegument density can be divided into three prominent regions: a penton-binding globular region, a helix-bundle stalk region, and a β-sheet-rich triplexbinding region. Fitting of the crystal structure of the truncated HSV-1 UL25 protein (the KSHV ORF19 homolog) and secondary structure analysis of the full-length ORF19 established that ORF19 constitutes the globular region with an N-terminal, 60-amino- acid-long helix extending into the stalk region. Matching secondary structural features resolved in the cryo-EM density with secondary structures predicted by sequence analysis identifies the triplex-binding region to be ORF32, a homolog of alphaherpesvirus UL17. Despite the high level of tegument structural similarities between KSHV and alphaherpesvirus, an ORF19 monomer in KSHV, in contrast to a UL25 dimer in alphaherpesviruses, binds each penton subunit, an observation that correlates with conformational differences in their pentons. This newly discovered organization of triplex-ORF32-ORF19 also resolves a longstanding mystery surrounding the virion location and conformation of alphaherpesvirus UL25 protein.