Modification of Daxx by small ubiquitin-related modifier-1

Small ubiquitin-related modifier-1 (SUMO-1) is a protein that is covalently modified to various cellular proteins and protects cells against both anti-Fas and TNF-induced cell death. Previously, we reported that the C-terminus of Daxx interacted with Ubc9, an E2 type SUMO-1 conjugating enzyme, as well as with SUMO-1. In BOSC23 cells expressing FLAG-Daxx together with HA-SUMO-1, 110 and 130kDa Daxx appeared and the 130kDa band bound to both anti-HA and anti-FLAG antibodies. This means that Daxx can be covalently modified by SUMO-1. Substitution of K630 and K631 abrogated the modification of Daxx by SUMO-1, implying that K630 and K631 were essential for sumoylation. Daxx (K630, 631A) and Daxx (K634, 636, 637A) in which the putative C-terminal nuclear localization signals (NLSs) were disrupted appeared in the nucleus, suggesting that the C-terminal NLS was not functional. Daxx (K630, 631A), the sumoylation defective mutant, was able to interact with PML and co-localized with PML in the PML oncogenic domains (PODs). Thus, our data show that sumoylation status of Daxx does not affect its presence in PODs.     

Overexpression of promyelocytic leukemia protein precludes the dispersal of ND10 structures and has no effect on accumulation of infectious herpes simplex virus 1 or its proteins

A key early event in the replication of herpes simplex virus 1 (HSV-1) is the localization of infected-cell protein no. 0 (ICP0) in nuclear structures knows as ND10 or promyelocytic leukemia oncogenic domains (PODs). This is followed by dispersal of ND10 constituents such as the promyelocytic leukemia protein (PML), CREB-binding protein (CBP), and Daxx. Numerous experiments have shown that this dispersal is mediated by ICP0. PML is thought to be the organizing structural component of ND10. To determine whether the virus targets PML because it is inimical to viral replication, telomerase-immortalized human foreskin fibroblasts and HEp-2 cells were transduced with wild-type baculovirus or a baculovirus expressing the M(r) 69,000 form of PML. The transduced cultures were examined for expression and localization of PML in mock-infected and HSV-1-infected cells. The results obtained from studies of cells overexpressing PML were as follows. (i) Transduced cells accumulate large amounts of unmodified and SUMO-I-modified PML. (ii) Mock-infected cells exhibited enlarged ND10 structures containing CBP and Daxx in addition to PML. (iii) In infected cells, ICP0 colocalized with PML in ND10 early in infection, but the two proteins did not overlap or were juxtaposed in orderly structures. (iv) The enlarged ND10 structures remained intact at least until 12 h after infection and retained CBP and Daxx in addition to PML. (v) Overexpression of PML had no effect on the accumulation of viral proteins representative of alpha, beta, or gamma groups and had no effect on the accumulation of infectious virus in cells infected with wild-type virus or a mutant (R7910) from which the alpha 0 genes had been deleted. These results indicate the following: (i) PML overexpressed in transduced cells cannot be differentiated from endogenous PML with respect to sumoylation and localization in ND10 structures. (ii) PML does not affect viral replication or the changes in the localization of ICP0 through infection. (iii) Disaggregation of ND10 structures is not an obligatory event essential for viral replication.     

The solution structure of the RING finger domain from the acute promyelocytic leukaemia proto-oncoprotein PML.

Acute promyelocytic leukaemia (APL) has been ascribed to a chromosomal translocation event which results in a fusion protein comprising the PML protein and the retinoic acid receptor alpha. PML is normally a component of a nuclear multiprotein complex (termed ND10, Kr bodies, nuclear bodies, PML oncogenic domains or PODs) which is disrupted in the APL disease state. PML contains a number of characterized motifs including a Zn2+ binding domain called the RING or C3HC4 finger. Here we describe the solution structure of the PML RING finger as solved by 1H NMR methods at physiological pH with r.m.s. deviations for backbone atoms of 0.88 and 1.39 A for all atoms. Additional biophysical studies including CD and optical spectroscopy, show that the PML RING finger requires Zn2+ for autonomous folding and that cysteines are used in metal ligation. A comparison of the structure with the previously solved equine herpes virus IE110 RING finger, shows significant differences suggesting that the RING motif is structurally diverse. The role of the RING domain in PML nuclear body formation was tested in vivo, by using site-directed mutagenesis and immunofluorescence on transiently transfected NIH 3T3 cells. Independently mutating two pairs of cysteines in each of the Zn2+ binding sites prevents PML nuclear body formation, suggesting that a fully folded RING domain is necessary for this process. These results suggest that the PML RING domain is probably involved in protein-protein interactions, a feature which may be common to other RING finger domains.     

Sumoylation of Daxx regulates IFN-induced growth suppression of B lymphocytes and the hormone receptor-mediated transactivation

Daxx has been shown to play an essential role in type I IFN-mediated suppression of B cell development and apoptosis. Recently, we demonstrated that Tyk2 is directly involved in IFN signaling for the induction and translocation of Daxx, which may result in growth arrest and/or apoptosis of B lymphocyte progenitors. To clarify the molecular mechanisms of how Daxx acts on growth suppression of B lymphocytes, we examined functions of a sumoylation-defective Daxx KA mutant (Daxx K630/631A), which substituted Lys 630 and Lys 631 to Ala. Importantly, Daxx KA localized in the cytoplasm, whereas wild-type Daxx localized in the nucleus. Murine pro-B cell line Ba/F3 expressing Daxx KA revealed a resistance to the IFN-induced growth suppression. It is noteworthy that treatment with an exportin inhibitor, leptomycin B, resulted in nuclear localization of Daxx KA and recovery of the IFN-induced growth suppression in Ba/F3 cells. Moreover, Daxx KA decreased the binding potential to promyelocytic leukemia protein (PML), and overexpression of PML recruited Daxx KA into PML oncogenic domains. Notably, a Daxx-small ubiquitin-related modifier fusion protein exhibited increased nuclear localization and ability to suppress cell growth in Ba/F3 cells. These results demonstrate that the IFN-induced growth suppression of B lymphocytes requires nuclear localization of Daxx through its sumoylation and proper interactions with PML.