Adenoviral E1B55K oncoprotein sequesters candidate leukemia suppressor sequence-specific single-stranded DNA-binding protein 2 into aggresomes

Abstract

Sequence-specific single-stranded DNA-binding protein 2 (SSBP2) is a candidate tumor suppressor for human acute myelogenous leukemia (AML). Inducible expression of SSBP2 causes growth arrest and partial differentiation in AML cells. Here, we report that the adenoviral oncoprotein E1B55K directly binds to endogenous SSBP2 protein and sequesters it into juxtanuclear bodies in adenovirally transformed human embryonic kidney (HEK) 293 cells. Similarly, transient expression of E1B55K in IMR90 fibroblasts and HeLa cells result in the formation of juxtanuclear bodies containing SSBP2. When nuclear export of E1B55K is prevented, SSBP2 remains associated with E1B55K in nuclear foci. A requirement for intact microtubules to retain the integrity of the juxtanuclear bodies suggests them to be E1B55K containing aggresomes. The adenoviral E1B55K protein has been shown to localize to the Mre11 complex and p53 to aggresome structures; together with the viral E4orf6 protein, E1B55K recruits a cellular E3 ubiquitin ligase that induces degradation of Mre11 and p53. However, our present studies reveal that E1B55K does not degrade SSBP2. These data demonstrate that E1B55K targets the candidate leukemia suppressor SSBP2 and suggest that subverting its function may contribute to cell transformation by viral oncoproteins.

Distinct Requirements of Adenovirus E1b55K Protein for Degradation of Cellular Substrates

“…The expression of E1b55K in cells can induce a large, perinuclear accumulation of the protein with characteristics of an aggresome (42). Many cellular proteins interacting with E1b55K have been observed to colocalize with E1b55K aggresomes (4,22,42,44,74). We therefore examined the ability of E1b55K mutant proteins to concentrate MRN and p53 into cytoplasmic aggregates (Fig.…”

“…This is likely to be the case, since the downregulation of all these substrates is prevented by the expression of transdominant cullin 5 (43) and by small interfering RNA knockdown of cullin 5 (R. A. Schwartz and M. D. Weitzman, unpublished data; 2, 43, 70). Other proteins that interact with E1b55K have been identified by genetic, biochemical, and proteomic studies (22,24,30,44,75); however, not all of these interacting proteins are targets for downregulation. It will be interesting to ascertain what determines the ubiquitination and degradation of E1b55K-associated proteins.…”

“…In transformed cells, E1b55K from Ad2 or Ad5 accumulates in a large cytoplasmic body (4,9,20,74) with characteristics of an aggresome (38,42). Many cellular proteins that interact with E1b55K localize at the aggresome in transfected and transformed cells (4,9,20,22,42,44,74). Therefore, protein aggregation has been proposed previously as a potential strategy utilized by E1b55K to inactivate cellular proteins (42) and promote transformation (31).…”

Self-association of adenovirus type 5 E1B-55 kDa as well as p53 is essential for their mutual interaction

“…It is therefore conceivable that the association with the MRN complex is also based on E1B-55 kDa self-association and the resulting increase in avidity. Other association partners of E1B-55 kDa, such as DNA ligase IV (Baker et al, 2007), Integrin a3 (Dallaire et al, 2009) or the single-stranded DNA-binding protein 2 (Fleisig et al, 2007), are not (yet) known to oligomerize but may possibly form similar complexes. p53 oligomerization may increase its avidity to associate with many binding partners p53 is known to associate with a large variety of cellular proteins, and our study on E1B-55 kDa raises the question whether the ability of p53 to form tetramers contributes to at least some of these associations.…”

Adenovirus E1B 55-Kilodalton Protein: Multiple Roles in Viral Infection and Cell Transformation

“…As a result of the interaction, the Ad protein is able to overcome growth inhibition by WT1. Similarly, it appears that the Ad5 E1B-55K protein localizes sequence-specific single-stranded DNA-binding protein 2 (SSBP2) to aggresomes in 293 cells (40). Interaction of the Ad5 E1B-55K protein with Daxx in 293 cell nuclei has also been reported (159).…”