Glucagon like-peptide-1 receptor is covalently modified by endogenous mono-ADP-ribosyltransferase.

Our previous study revealed a mono-ADP-ribosyltransferase mediated in vitro mono-ADP-ribosylation of IC(3) peptide, a peptide with sequence corresponded to third intracellular loop of glucagon like-peptide-1 (GLP-1) receptor. Furthermore, Arg(348) was shown to be modified amino acid residue although its mutation did not eliminate mono-ADP-ribosylation completely. In order to further study the signaling mechanisms of GLP-1 receptor, we took on lease a possibility that an alternative site of enzymatic modification exist so mono-ADP-ribosylation of Cys(341) was hypothesized. The results confirmed both Arg(348) and Cys(341) as a site of mono-ADP-ribosylation where Arg(348) is modified predominantly. Sum of mono-ADP-ribosylation rate of both single IC(3) mutants coincided with IC(3) rate. What is in vivo role of Cys(341) mono-ADP-ribosylation is entirely speculative but our study represents an important step toward a complete understanding of signaling via GLP-1 receptor.