REVERSE TRANSCRIPTION-ASSOCIATED DEPHOSPHORYLATION OF HEPADNAVIRUS NUCLEOCAPSID

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Hepatitis B Virus (HBV) is a major global health problem, with currently more than 300 million chronically infected worldwide. Its extremely high degree of infectivity has been in part linked to the nature of its assembly and morphogenesis. Dynamic protein phosphorylation is paramount to the regulation of viral, as well as normal, cellular processes, although it presents numerous challenges for MS study due to the transience and labile nature of the phosphate modification. Phosphorylation of the HBV capsid protein may regulate its numerous functional roles in the HBV lifecycle, including, during morphogenesis, transmitting the viral maturation signal that triggers envelopment and secretion of mature infectious virions. To identify biophysical and biochemical correlates of viral maturation, we have characterized the gross structure of purified capsids from three stages of viral maturation by atomic force microscopy (AFM), and the phosphorylation of the capsid protein at these stages by MALDI-TOF MS ESI-Q-oTOF MS/MS, and vibrational cooling (VC) MALDI-FT MS, which minimizes phosphate loss. Duck HBV (DHBV) nucleocapsids from three extremes of viral maturation (immature, RNA-containing; mature, DNA-containing; and virion-derived nucleocapsids) were isolated from virus-expressing cell lines and purified to homogeneity by gradient ultracentrifugation. Intact nucleocapsids were spotted directly onto AFM targets and visualized by AFM. We have previously reported that, using MALDI-TOF MS, ESI nanospray and LC/MSn and VC MALDI-FTICR MS, we were able to detect a novel DHBV capsid phosphopeptide, then obtain phosphosite localization (to S230) by conducting successive SORI-CAD experiments (MS2, MS3) upon it. Furthermore, we obtained complete sequencing and phosphosite localization for a novel DHBV capsid pentaphosphorylated peptide using ESI-Q-oTOF MS/MS, confirming four known sites of capsid phosphorylation (T239, S245, S257, and S259) and identifying a second novel phosphorylation site (S232). After the recent installation of the LTQ-Orbitrap MS, we have begun nanoLC/MS/MS analysis of the capsid digests, to search for further phosphosides and other PTMs and to identify low-abundance proteins.