Decapping scavenger (DcpS) enzymes catalyze the cleavage of a residual cap 
structure following 3' → 5' mRNA decay. Some previous studies suggested that 
both m(7)GpppG and m(7)GDP were substrates for DcpS hydrolysis. Herein, we show 
that mononucleoside diphosphates, m(7)GDP (7-methylguanosine diphosphate) and 
m(3)(2,2,7)GDP (2,2,7-trimethylguanosine diphosphate), resulting from mRNA 
decapping by the Dcp1/2 complex in the 5' → 3' mRNA decay, are not degraded by 
recombinant DcpS proteins (human, nematode, and yeast). Furthermore, whereas 
mononucleoside diphosphates (m(7)GDP and m(3)(2,2,7)GDP) are not hydrolyzed by 
DcpS, mononucleoside triphosphates (m(7)GTP and m(3)(2,2,7)GTP) are, 
demonstrating the importance of a triphosphate chain for DcpS hydrolytic 
activity. m(7)GTP and m(3)(2,2,7)GTP are cleaved at a slower rate than their 
corresponding dinucleotides (m(7)GpppG and m(3)(2,2,7)GpppG, respectively), 
indicating an involvement of the second nucleoside for efficient DcpS-mediated 
digestion. Although DcpS enzymes cannot hydrolyze m(7)GDP, they have a high 
binding affinity for m(7)GDP and m(7)GDP potently inhibits DcpS hydrolysis of 
m(7)GpppG, suggesting that m(7)GDP may function as an efficient DcpS inhibitor. 
Our data have important implications for the regulatory role of m(7)GDP in mRNA 
metabolic pathways due to its possible interactions with different cap-binding 
proteins, such as DcpS or eIF4E.