The degradation of most eukaryotic mRNAs is initiated by removal of the poly(A) 
tail, and the major deadenylase activity is associated with the CCR4/CAF1/NOT 
complex (NOT complex). We here study the role of CNOT10, a protein that is found 
in human and trypanosome, but not in yeast, NOT complexes. Trypanosome (Tb) 
CNOT10 is essential for growth. TbCNOT10 interacted with the deadenylase TbCAF1 
and the scaffold protein TbNOT1; TbCAF1 also interacted with TbNOT1 in a yeast 
two-hybrid assay. In both trypanosomes and human embryonic kidney cells, 
approximately half of CAF1 was associated with the NOT complex. Depletion of 
CNOT10 from human cells did not affect this association. In contrast, depletion 
of TbCNOT10 in trypanosomes caused a decrease in the level of TbNOT1, detachment 
of TbCAF1 from the complex and pronounced stabilization of most trypanosome 
mRNAs. Artificial tethering of TbCAF1 to a reporter mRNA in vivo resulted in 
mRNA degradation, and this was not affected by TbCNOT10 depletion. We conclude 
that in trypanosomes, TbCNOT10 may stabilize the interaction between TbCAF1 and 
the NOT complex. The results further suggest that TbCAF1 is only able to 
deadenylate mRNA in vivo if it is recruited to the mRNA through other NOT 
complex components.