1. Schematic diagram of ubiquitin and SUMO modification pathways
pathways. Ubiquitin (Ub, top pathway) and SUMO (S, bottom), known
as modifiers, are small polypeptides that are usually conjugated to internal
lysine residues of target proteins through their C-terminal glycine residues,
forming isopeptide bonds. Both modifications are reversible. Modification
can be by a single modifier or by a chain of covalently linked modifiers
(polyubiquitylation or polySUMOylation). The number of enzymes in
the yeast Saccharomyces cerevisiae and in humans, respectively, are shown
in parentheses. UBPs, ubiquitin-specific proteases; ULPs, ubiquitin-like
protein (SUMO)-specific proteases.
small polypeptides that are usually conjugated to internal
lysine residues of target proteins through their C-terminal glycine residues, forming isopeptide bonds

2. THE UBIQUITIN–PROTEASOME PATHWAY
Degradation of the tagged substrate by the 26S proteasome
The ubiquitin–proteasome pathway. (1) Activation of ubiquitin by E1. (2) Transfer of activated ubiquitin from E1 to a member of the E2 family. (3) Transfer of activated ubiquitin from E2 to a substrate-specific E3. (4) Formation of a substrate–E3 complex and biosynthesis of a substrate-anchored polyubiquitin chain. (5) Binding of the polyubiquitinated substrate to the ubiquitin receptor subunit in the 19S complex of the 26S proteasome and degradation of the substrate to short peptides by the 20S complex. (6) Recycling of ubiquitin via the action of isopeptidases.
conjugation of ubiquitin to the target molecule

3. HIERARCHICAL STRUCTURE OF THE UBIQUITIN-CONJUGATING MACHINERY
. A single E1 catalyzes activation of ubiquitin and transfers it to several E2 enzymes. In most cases, an E2 transfers ubiquitin to several E3s, while in a few cases the E2 is E3-specific. E3s can be substrate-specific or can recognize several substrates via similar, but not identical motifs. Certain substrates can be targeted by several E3s, probably via distinct recognition motifs.

4. MODES OF RECOGNITION OF PROTEIN SUBSTRATES BY DIFFERENT E3S
. An E3 can recognize a substrate constitutively via a primary motif such as the N-terminal residue (N-end rule). Many proteins are recognized following post-translational modification (e.g. phosphorylation) or association with an ancillary protein (e.g. Hsc or HPV-E6). N-R, N-terminal receptor.

5. MODES OF RECOGNITION OF PROTEIN SUBSTRATES BY DIFFERENT E3S.
An E3 can recognize a substrate constitutively via a primary motif such as the N-terminal residue (N-end rule). Many proteins are recognized following post-translational modification (e.g. phosphorylation) or association with an ancillary protein (e.g. Hsc or HPV-E6). N-R, N-terminal receptor.

6. Small Ubiquitin-like Modifier or SUMO proteins are a family of small proteins that are covalently attached to and detached from other proteins in cells to modify their function.
SUMOylation is a post-translational modification involved in various cellular processes, such as nuclear-cytosolic transport, transcrptional regulation, apoptosis, protein stability, response to stress, and progression through the cell cycle
SUMO proteins are similar to ubiquitin, and SUMOylation is directed by an enzymatic cascade analogous to that involved in ubiquitination. In contrast to ubiquitin, SUMO is not used to tag proteins for degradation

7. XPC may participate in BER of GT mismatches ontributing to the supression of spontaneous mutations that may be one of the factors promoting carcinogenesis in xeroderma pigmentosum
Bergink and Jantsch Nature, 2009