Might act as an E3 ubiquitin-protein ligase. Appears to be required for normal cell-type proportioning and cell sorting during multicellular development. In addition to being necessary for a normal percentage of prestalk cells and the organization of the slug, rbrA is also necessary for spore cell viability. [E2 ubiquitin-conjugating enzyme]-S-ubiquitinyl-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-N(6)-ubiquitinyl-L-lysine. Protein modification; protein ubiquitination. Members of the RBR family are atypical E3 ligases. They interact with the E2 conjugating enzyme UBE2L3 and function like HECT-type E3 enzymes: they bind E2s via the first RING domain, but require an obligate trans-thiolation step during the ubiquitin transfer, requiring a conserved cysteine residue in the second RING domain. Prestalk cell numbers are reduced in slugs and these prestalk cells do not localize to the tip of slugs. Development terminates at the finger or slug stage, and these slugs do not phototax, possibly because the tip region does not properly form. Belongs to the RBR family.