Biology Reference
In-Depth Information
protein complexes accumulate a limited amount of ND10-associated proteins, i.e.,
smaller incorporated reiterations had smaller dot size. The dot size did not increase
with interferon upregulation of PML or Sp100, indicating these sites were not
ND10 nucleating sites. Rather they are new sites of DNA or chromatin that bound
PML or Sp100 and entered the visual range because of the reiteration of binding
sites. Since most HCMV viral genomes have no visually recognizable PML
attached even after entering the nucleus (see Fig. 1b), except for the few particles
at ND10 that transcribe, this virus should be re-evaluated for association with
ND10. The key experiment would be to determine whether the transcribing viruses
are at ND10 and whether such apparent ND10-associated aggregates had moved or
were newly formed. Because transcribing genomes cannot presently be labeled in
a time-resolved manner, we may use immediate early transcript environment
formation as a surrogate, by tracking the accumulation of GFP-tagged IE2 in cells
containing Cherry-tagged ND10.
Structural and Functional Aspects of IE1
IE1 is the viral protein that disperses ND10 (Korioth et al. 1996; Ahn and Hayward
1997; Ishov et al. 1997; Wilkinson et al. 1998). In contrast to the immediate early
protein ICP0 of HSV-1, it does not do so by the proteosome-dependent destruction
of PML and Daxx. It was suggested that IE1 disperses ND10 by binding to constitu-
ents and their removal from ND10 over time, because IE1 and PML interact (Ahn
et al. 1998). Neither the mechanism of interaction nor the means of dispersal are
known. One preliminary finding on MCMV IE1 casts doubt on the idea that
stochastic removal of PML by IE1 is the sole mechanism. Removing a 7-amino acid
sequence from one of the helices in the N-terminal region of MCMV IE1 eliminates
the dispersive function but not the binding function (Q. Tang and G. Maul, unpub-
lished data). We may have to search for a mechanism that includes a function other
than simple binding of PML, for dispersion of ND10. We also may have to compare
IE1 of HCMV and MCMV; such a comparison could help illuminate the different
domains necessary for certain functions.
IE1 has several functional properties that have been used to probe its structure
through mutational analysis. It augments viral and host gene transcription, dis-
perses ND10 and binds repressor proteins. These functions are driven either by the
indirect augmentation of transcription, possibly by alleviating repression of IE1
binding to p107 (Poma et al. 1996) or HDAC (Tang and Maul 2003; Nevels et al.
2004b), or by the direct augmentation of transcription through IE1 binding of
transcription factors (Lukac et al. 1997). Determining how these proteins bind to
IE1, and to which interface, is important for developing effective interference
strategies. Because IE1 plays an important role during low-particle infections
(assumed to be the normal infection mode), a strategy aimed at IE1 inactivation
might be successful in blocking the HDAC-binding capacity of IE1 and allowing
the host cell to silence competent viral genomes.
