DNA methylation, an essential regulator of transcription and chromatin 
structure, is established and maintained by the coordinated action of three DNA 
methyltransferases: DNMT1, DNMT3A and DNMT3B, and the inactive accessory factor 
DNMT3L. Disruptions in DNMT3B function are linked to carcinogenesis and genetic 
disease. DNMT3B is also highly alternatively spliced in a tissue- and 
disease-specific manner. The impact of intra-DNMT3 interactions and alternative 
splicing on the function of DNMT3 family members remains unclear. In the present 
work, we focused on DNMT3B. Using a panel of in vitro assays, we examined the 
consequences of DNMT3B splicing and mutations on its ability to bind DNA, 
interact with itself and other DNMT3's, and methylate DNA. Our results show 
that, while the C-terminal catalytic domain is critical for most DNMT3B 
functions, parts of the N-terminal region, including the PWWP domain, are also 
important. Alternative splicing and domain deletions also influence DNMT3B's 
cellular localization. Furthermore, our data reveal the existence of extensive 
DNMT3B self-interactions that differentially impact on its activity. Finally, we 
show that catalytically inactive isoforms of DNMT3B are capable of modulating 
the activity of DNMT3A-DNMT3L complexes. Our studies therefore suggest that 
seemingly 'inactive' DNMT3B isoforms may influence genomic methylation patterns 
in vivo.