THE MOUSE AND HUMAN GENES ENCODING THE RECOGNITION COMPONENT OF THE N-END RULE PATHWAY

The N-end rule relates the in vivo half-life of a protein to the ident ity of its N-terminal residue. The N-end rule pathway is one proteolyt ic pathway of the ubiquitin system. The recognition component of this pathway, called N-recognin or E3, binds to a destabilizing N-terminal residue of a substrate protein and participates in the formation of a substrate-linked multiubiquitin chain. We report the cloning of the mo use and human Ubr1 cDNAs and genes that encode a mammalian N-recognin called E3 alpha. Mouse UBR1p (E3 alpha) is a 1,757-residue (200-kDa) p rotein that contains regions of sequence similarity to the 225-kDa Ubr 1p of the yeast Saccharomyces cerevisiae. Mouse and human UBR1p have a pparent homologs in other eukaryotes as well, thus defining a distinct family of proteins, the UBR family. The residues essential for substr ate recognition by the yeast Ubr1p are conserved in the mouse UBR1p. T he regions of similarity among the UBR family members include a putati ve zinc finger and RING-H2 finger, another zinc-binding domain. Ubr1 i s located in the middle of mouse chromosome 2 and in the syntenic 15q1 5-q21.1 region of human chromosome 15. Mouse Ubr1 spans approximate to 120 kilobases of genomic DNA and contains approximate to 50 exons. Ub r1 is ubiquitously expressed in adults, with skeletal muscle and heart being the sites of highest expression. In mouse embryos, the Ubr1 exp ression is highest in the branchial arches and in the tail and limb bu ds. The cloning of Ubr1 makes possible the construction of Ubr1-lackin g mouse strains, a prerequisite for the functional understanding of th e mammalian N-end rule pathway.