Title : Rumi functions as both a protein O-glucosyltransferase and a protein O-xylosyltransferase
Abstract :
Mutations in rumi result in a temperature-sensitive loss of Notch signaling in Drosophila
Drosophila Rumi is a soluble, endoplasmic reticulum-retained protein with a CAP10 domain that functions as a protein O-glucosyltransferase
In human and mouse genomes, three potential Rumi homologues exist: one with a high degree of identity to Drosophila Rumi (52%), and two others with lower degrees of identity but including a CAP10 domain ( KDELC1 and KDELC2 )
Here we show that both mouse and human Rumi , but not KDELC1 or KDELC2 , catalyze transfer of glucose from UDP-glucose to an EGF repeat from human factor VII
Similarly, human Rumi , but not KDELC1 or KDELC2 , rescues the Notch phenotypes in Drosophila rumi clones
During characterization of the Rumi enzymes , we noted that, in addition to protein O-glucosyltransferase activity, both mammalian and Drosophila Rumi also showed significant protein O-xylosyltransferase activity
Rumi transfers Xyl or glucose to serine 52 in the O-glucose consensus sequence ( ) of factor VII EGF repeat
Surprisingly, the second serine ( S53 ) facilitates transfer of Xyl, but not glucose, to the EGF repeat by Rumi
EGF16 of mouse Notch2 , which has a diserine motif in the consensus sequence ( ), is also modified with either O-Xyl or O-glucose glycans in cells
Mutation of the second serine (S590A) causes a loss of O-Xyl but not O-glucose at this site
Altogether, our data establish dual substrate specificity for the glycosyltransferase Rumi and provide evidence that amino acid sequences of the recipient EGF repeat significantly influence which donor substrate (UDP-glucose or UDP-Xyl) is used