E4 ligase

Fig. 5. Domain structure of Hsp70 co-chaperones. Individual domains/modules are represented by differendy shaded boxes. The known structural features and functions of domains are indicated. The following abbreviations for the different domains were used NLS, nuclear localization sequence TRSEEX, Thr-Arg-Ser-Glu-Glu-Xaa repeat motif Ub, ubiquitin-like domain Bag, Bag homology region WW, Trp-Trp domain TPR, tetratricopeptide repeat GGMP, Gly-Gly-Met-Pro repeat motif +/—, charged region U box, U box motif of E4 ubiquitin ligases DnaK, interaction site for DnaK 70, interaction site for Hsp70 90, interaction site for Hsp90.

After the linkage of Ub to the substrate protein, a polyubiquitin (multiubiquitin) chain is often formed, in which the C-terminus of each ubiquitin unit is linked to a specific Lys residue (most commonly Lys48) of the previous Ub. The multiubiquitin-chain assembly is a processive reaction that usually requires only El, E2 and E3. However, an efficient multiubiquitination needs an additional conjugation factor termed E4 enzyme (Hoppe, 2005). Ubiquitin-protein ligases are, directly or indirectly, those that bind specific protein substrates, promote the transfer of Ub, and form a thioester intermediate to amide linkages with proteins or polyubiquitin chains. 

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