Table I.

Summary of UBC domain-containing proteins, their nomenclature, and activity as ubiquitin E2 enzymes

Previously unnamed UBC domain-containing Arabidopsis proteins were given UBC numbers and grouped together based on sequence similarity (see Fig. 1). InsolubleX, 6×His-tagged protein insoluble when expressed in E. coli; InsolubleY, 6×His-tagged protein insoluble when expressed in either E. coli or insect cells; InsolubleZ, UBC contains predicted transmembrane domain C terminal to UBC domain and protein is insoluble when expressed with predicted transmembrane domain, but soluble when expressed without it. References (as indicated in table by superscript numbers): 1, Sullivan and Vierstra, 1993; 2, Girod et al., 1993; 3, Van Nocker et al., 1996; 4, Bartling et al., 1993; 5, Criqui et al., 2002; 6, Yanagawa et al., 2004; 7, this study; 8, Girod and Vierstra, 1993; and 9, Van Nocker and Vierstra, 1993.

Group No.	Gene UBC	AGI Code	Active in TE^a	Active in Ub^b	Comments	GenBank Accession No.
III	1	At1g14400	Yes^c ^d^,1,7	Yes^1,8,7	E3-in^e^,7, E3-dep^f^,1,8	DQ027016
III	2	At2g02760	Yes⁷	Yes⁷	E3-in⁷	DQ027017
III	3	At5g62540	nd^g	Yes⁷	E3-in⁷	DQ027018
IV	4	At5g41340	Yes^h,8,9	Yes⁷	E3-in⁷, E3-dep⁷	DQ027019
IV	5	At1g63800	nd	Yes⁷	E3-in⁷	DQ027020
IV	6	At2g46030	nd	Yes⁷	E3-in⁷	DQ027021
V	7	At5g59300	Yes^h^,3	nd
V	13	At3g46460	Yes^h^,3	nd		DQ027027
V	14	At3g55380	Yes^h^,3	nd		DQ027028
VI	8	At5g41700	Yes^h^,2,^d^,7	Yes^2,7	E3-dep⁷	DQ027022
VI	9	At4g27960	Yes^d^,6	nd		DQ027023
VI	10	At5g53300	Yes⁷	Yes⁷	E3-dep⁷	DQ027024
VI	11	At3g08690	Yes⁷	Yes⁷	E3-dep⁷	DQ027025
VI	12	At3g08700	nd	nd	InsolubleX⁷	DQ027026
VI	28	At1g64230	Yes⁷	Yes⁷	E3-dep⁷	DQ027041
VI	29	At2g16740	nd	Yes⁷	E3-dep⁷	DQ027042
VI	30	At5g56150	nd	Yes⁷	E3-dep⁷	DQ027043
VII	15	At1g45050	Yesⁱ^,4	No^j^,7		DQ027029
VII	16	At1g75440	nd	No⁷		DQ027030
VII	17	At4g36410	nd	No⁷		DQ027031
VII	18	At5g42990	nd	No⁷		DQ027032
VIII	19	At3g20060	Yes^d^,5,7	No		DQ027033
VIII	20	At1g50490	nd	No		DQ027034
IX	21	At5g25760	nd	nd	InsolubleY⁷	DQ027035
X	22	At5g05080	nd	Yes⁷	E3-in⁷	DQ027036
XI	23	At2g16920	nd	nd	Not cloned
XI	24	At2g33770	nd	nd	InsolubleY⁷	DQ027037
XI	25	At3g15355	nd	nd	InsolubleY⁷	DQ027038
XI	26	At1g53020	nd	No⁷		DQ027039
XII	27	At5g50870	Yes⁷	No⁷		DQ027040
XIII	31	At1g36340	nd	nd	InsolubleX⁷	DQ027044
XIV	32	At3g17000	Yes⁷	nd	InsolubleZ⁷	DQ027045
XIV	33	At5g50430	nd	nd	InsolubleZ⁷	DQ027046
XIV	34	At1g17280	nd	Yes⁷	E3-dep⁷, InsolubleZ⁷	DQ027047
XV	35	At1g78870	nd	Yes⁷	E3-dep⁷	DQ027048
XV	36	At1g16890	Yes⁷	Yes⁷	E3-dep⁷	DQ027049
XVI	37	At3g24515	nd	No⁷		DQ027050

↵a A thioester linkage between the E2 and ubiquitin was observed (examples shown in Fig. 2).
↵b Active in ubiquitination and indicates the E2 showed activity in the transfer of ubiquitin to a peptide linkage (see Fig. 4).
↵c Yes, Activity has been demonstrated.
↵d Ub-UBC conjugate eliminated under sulfhydryl-reducing conditions indicating Ub-E2 thioester linkage, as shown in reference.
↵e E3-in, Ubiquitination of the E2 or other proteins that occurs in the absence of added E3 for E3-independent activity (for an E2 with strong activity, see Fig. 3).
↵f E3-dep, Ubiquitination that requires E3 for E3-dependent (for examples, see Fig. 4).
↵g nd, Protein was not assayed for activity.
↵h Ub, E2 conjugate eliminated under sulfhydryl-reducing conditions, indicating Ub-E2 thioester linkage, but data not shown in reference.
↵i Ub-E2 conjugate not eliminated after incubation under sulfhydryl-reducing conditions, but with 100 mm Lys.
↵j No, No activity was detected.