The microRNA Machinery.

MicroRNAs (miRNAs) are short (~22 nucleotides) single-stranded RNA molecules that primarily function to negatively regulate gene expression at the post-transcriptional level. miRNAs have thus been implicated in the regulation of a wide variety of normal cell functions and pathophysiological conditions. The miRNA machinery consists of a series of protein complexes which act to: (1) cleave the precursor-miRNA hairpin from its primary transcript (i.e. DROSHA and DGCR8); (2) traffic the miRNA hairpin between nucleus and cytoplasm (i.e. XPO5); (3) remove the loop sequence of the hairpin by a second nucleolytic cleavage reaction (i.e. DICER1); (4) facilitate loading of the mature miRNA sequence into an Argonaute protein (typically AGO2) as part of the RNA-Induced Silencing Complex (RISC); (5) guide the loaded RISC complex to complementary, or semi-complementary, target transcripts and (6) facilitate gene silencing via one of several possible mechanisms.