... lacking Lys48 (UbR48) orLys63 (UbR63), we found that ubiquitin chain synthe-sis on Rac1L61 was markedly but not completelyinhibited by UbR48 and that, by contrast, UbR63 sup-ported the formation ... apparently not used forubiquitin addition. Therefore, the reason why Lys147rather than Lys5 is the main acceptor site for ubiquitinin Rac1 remains to be elucidated. Of note, however, is that ... Rac1 are compared (supplemen-tary Fig. S4), Lys147 has closely similar spatialsituations; this observation raises the hypothesis that similar mechanisms and homologous E3 ubiquitinligases are...