E3-- E3 proteins (or ubiquitin-protein ligases) are the last components in the ubiquitin (Ub) conjugation cascade and are generally considered to be most important in controlling target specificity. E3s recruit the target protein, position it for optimal transfer of the Ub moiety from the E2 to a lysine residue in the target protein, and initiate the conjugation.

E3s can be either monomeric proteins or multimeric complexes. E3s are the most numerous and diverse factors involved in the ubiquitin conjugation cascade. Four main types of E3s have been described, based on subunit composition and mechanism of action: HECT, RING/U-Box, Cullin (CUL)-based and APC. In most or all cases, these domains appear to serve as the interaction site with the E2 enzymes.

E3:FBOX-- F-box protein is one of the four subunits in SCF class of E3. The conserved 60–amino acid F-box motif is located at the N terminus of the protein in almost all F-box proteins and is responsible for binding to the ASK/SKP subunit in the SCF E3 complex. F-box protein also contains protein–protein interaction domains required for substrate binding.

E3:FBOX:C4-- FBOX sub-family C4.

  1 MTSDALTIPS ELESALRLRT VQYFITKRPW LDLYGVHVRP VPPFGSTSRK PHFDPALIHR
 61 CLPDELLFEV FARMMPYDLG RASCVCRKWR YTVRNPMFWR NACLKAWQTA GVIENYKILQ
121 SKYDGSWRKM WLLRSRVRTD GLYVSRNTYI RAGIAEWKIT NPVHIVCYYR YIRFYPSGRF
181 LYKNSSQKLK DVAKYMNFKA SKSENLYKGT YTLSMSDDKI EAAVLYPGTR PTVLRIRLRL
241 RGTAIGANNR MDLLSLVTSG VNDEEISSTE EDILGLVEGW EDDETHNPDI PAVSHKRGMT
301 AFVFVPFEEV DESVLNLPPE KMDYYVTG