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Journal of Virology, March 1999, p. 2181-2192, Vol. 73, No. 3
0022-538X/99/$00.00+0
Capsid Structure of Simian Cytomegalovirus from
Cryoelectron Microscopy: Evidence for Tegument Attachment
Sites
Benes L.
Trus,1,2
Wade
Gibson,3
Naiqian
Cheng,1 and
Alasdair
C.
Steven1,*
Laboratory of Structural Biology,
NIAMS,1 and
Computational Bioscience and
Engineering Laboratory, CIT,2 National
Institutes of Health, Bethesda, Maryland 20892, and
Virology
Laboratories, Johns Hopkins School of Medicine, Baltimore, Maryland
212053
Received 9 September 1998/Accepted 16 November 1998
We have used cryoelectron microscopy and image reconstruction to
study B-capsids recovered from both the nuclear and the cytoplasmic fractions of cells infected with simian cytomegalovirus (SCMV). SCMV,
a representative betaherpesvirus, could thus be compared with
the previously described B-capsids of the alphaherpesviruses, herpes simplex virus type 1 (HSV-1) and equine herpesvirus 1 (EHV-1), and of channel catfish virus, an evolutionarily remote herpesvirus. Nuclear B-capsid architecture is generally conserved with SCMV, but it
is 4% larger in inner radius than HSV-1, implying that its ~30%
larger genome should be packed more tightly. Isolated SCMV B-capsids
retain a relatively well preserved inner shell (or "small core") of
scaffolding-assembly protein, whose radial-density profile
indicates that this protein is ~16-nm long and consists of two
domains connected by a low-density linker. As with HSV-1, the hexons
but not the pentons of the major capsid protein (151 kDa) bind the
smallest capsid protein (~8 kDa). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed cytoplasmic B-capsid
preparations to contain proteins similar in molecular weight to the
basic phosphoprotein (~119 kDa) and the matrix proteins (65 to 70 kDa). Micrographs revealed that these particles had variable amounts of
surface-adherent material not present on nuclear B-capsids that we take
to be tegument proteins. Cytoplasmic B-capsids were classified
accordingly as lightly, moderately, or heavily tegumented. By comparing
the three corresponding density maps with each other and with the
nuclear B-capsid, two interactions were identified between
putative tegument proteins and the capsid surface. One is between the
major capsid protein and a protein estimated by electron microscopy to
be 50 to 60 kDa; the other involves an elongated molecule
estimated to be 100 to 120 kDa that is anchored on the triplexes, most
likely on its dimer subunits. Candidates for the proteins bound at
these sites are discussed. This first visualization of such linkages
makes a step towards understanding the organization and functional
rationale of the herpesvirus tegument.
*
Corresponding author. Mailing address: Laboratory of
Structural Biology, NIAMS, Bldg. 6, Rm. B2-34, 6 Center Dr., MSC 2717, National Institutes of Health, Bethesda, MD 20892-2717. Phone: (301)
496-0132. Fax: (301) 480-7629. E-mail:
Alasdair_Steven{at}nih.gov.
Journal of Virology, March 1999, p. 2181-2192, Vol. 73, No. 3
0022-538X/99/$00.00+0
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