Table 2.

Viruses and ubiquitin-independent protein degradation by proteasomes

Virus class	Degradation inducer	Target substrate	Absence of ubiquitination	Unstable in ts20 cells	Immune to cDN ubiquitin	Unstable lysine-less mutant	In vitro degradation	Compelling arguments for ubiquitin- independent degradation?	Proposed mechanisms	References
aDS-DNA viruses	HPV E6	p53		√	√			Yes	20S dCP recognition of p53 upon C-terminus binding to E6	67
	HCMV pp71	Daxx		√				Yes	pp71-induced substrate binding to 19S eRP	98, Winkler and Kalejta (manuscript in prep)
	HCMV pp71	Rb, p107, p130	√	√	√			Yes	pp71-induced substrate binding to 19S RP	97, Winkler and Kalejta (manuscript in prep)
	?	HCMV pUL21a	√	√		√		Yes	20S CP recognition of unstructured pUL21a C-terminal domain	103
	?	MCMV M141	√	√				Yes	C-terminal fPEST domain as degradation signal	105
bSS-DNA viruses	?	MVM NS2	√	√	√			Yes	20S CP recognition of NS2 C- termius	111
Reverse transcribing viruses	?	HBV X				√		Not yet	20S CP binding; Chaperone Hdj1 participation	119-123
	p14ARF	HIV Tat				√		Not yet	20S CP binding; PA28γ participation	133, 135-136
	HTLV Tax	IκBα				√		Not yet	Tax-mediated recruitment of IκBa to 20S CP	148-149
Postive strand RNA viruses	?	HCV core				√		Yes	20S CP binding; PA28γ participation	156
	?	HCV F		√		√	√	Yes	20S CP recognition of F (aa 20- 60)	158

^aDS-DNA: double stranded DNA

^bSS-DNA: single stranded DNA

^cDN: dominant-negative

^dCP: core particle

^eRP: regulartory particle

^fPEST: proline (P), glutamic acid (E), serine (S), and threonine (T).