Caspases cleave and inhibit the microRNA processing protein DiGeorge Critical Region 8

DGCR8 (DiGeorge Critical Region 8) is an essential microRNA (miRNA) processing protein that recognizes primary transcripts of miRNAs (pri-miRNAs) and triggers their cleavage by the Drosha nuclease. We previously found that Fe(III) heme binds and activates DGCR8. Here we report that in HeLa cells, DGCR8 undergoes two proteolytic events that produce two C-terminal fragments called DGCR8C1 and DGCR8C2, respectively. DGCR8C2 accumulates during apoptosis and is generated through cleavage by a caspase. The caspase cleavage site is located in the central loop of the heme-binding domain. Cleavage of DGCR8 by caspase-3 in vitro results in loss of the otherwise tightly bound Fe(III) heme cofactor, dissociation of the N- and C-terminal proteolytic fragments, and inhibition of the pri-miRNA processing activity. These results reveal an intrinsic mechanism in the DGCR8 protein that seems to have evolved for regulating miRNA processing via association with Fe(III) heme and proteolytic cleavage by caspases. Decreased expression of miRNAs has been observed in apoptotic cells, and this change was attributed to caspase-mediated cleavage of a down-stream miRNA processing nuclease Dicer. We suggest that both the Drosha and Dicer cleavage steps of the miRNA maturation pathway may be inhibited in apoptosis and other biological processes where caspases are activated.