Several mammalian ubiquitin carrier proteins, but not E2(20K), are related to the 20-kDa yeast E2, RAD6.

Western blot analysis was used to probe the relationships between the multiple ubiquitin carrier proteins (E2 s) of rabbit reticulocytes and the 20-kDa E2 encoded by the RAD6 gene of the yeast S. cerevisiae. Reticulocyte E2-14K, E2-17K, and E2-25K each reacted with two or more polyclonal anti-RAD6 antibody preparations; E2-20K, E2-35K, and E2-230K did not cross-react. These results suggest that some, but not all, reticulocyte E2 s are members of a RAD6-like protein family which is conserved within and across species. RAD6 and E2-20K were also shown to multi-ubiquitinate histones by different mechanisms.