Histone H3 lysine 4 (K4) methylation is a prevalent mark associated with 
transcription activation and is mainly catalyzed by the MLL/SET1 family histone 
methyltransferases. A common feature of the mammalian MLL/SET1 complexes is the 
presence of three core components (RbBP5, Ash2L and WDR5) and a catalytic 
subunit containing a SET domain. Unlike most other histone lysine 
methyltransferases, all four proteins are required for efficient H3 K4 
methylation. Despite extensive efforts, mechanisms for how three core components 
regulate MLL/SET1 methyltransferase activity remain elusive. Here we show that a 
heterodimer of Ash2L and RbBP5 has intrinsic histone methyltransferase activity. 
This activity requires the highly conserved SPRY domain of Ash2L and a short 
peptide of RbBP5. We demonstrate that both Ash2L and the MLL1 SET domain are 
capable of binding to S-adenosyl-L- [methyl-(3)H] methionine in the MLL1 core 
complex. Mutations in the MLL1 SET domain that fail to support overall H3 K4 
methylation also compromise SAM binding by Ash2L. Taken together, our results 
show that the Ash2L/RbBP5 heterodimer plays a critical role in the overall 
catalysis of MLL1 mediated H3 K4 methylation. The results we describe here 
provide mechanistic insights for unique regulation of the MLL1 methyltransferase 
activity. It suggests that both Ash2L/RbBP5 and the MLL1 SET domain make direct 
contacts with the substrates and contribute to the formation of a joint 
catalytic center. Given the shared core configuration among all MLL/SET1 family 
HMTs, it will be interesting to test whether the mechanism we describe here can 
be generalized to other MLL/SET1 family members in the future.