LIM-domain proteins, LIMD1, Ajuba, and WTIP are required for microRNA-mediated gene silencing

In recent years there have been major advances with respect to the identification of the protein components and mechanisms of micro-RNA (miRNA) mediated silencing. However, the complete and precise repertoire of components and mechanism(s) of action remain to be fully elucidated. Herein we reveal the identification of a family of three LIM domain-containing proteins, LIMD1, Ajuba and WTIP (Ajuba LIM proteins) as novel mammalian processing body (P-body) components, which highlight a novel mechanism of miRNA-mediated gene silencing. Furthermore, we reveal that LIMD1, Ajuba, and WTIP bind to Ago1/2, RCK, Dcp2, and eIF4E in vivo, that they are required for miRNA-mediated, but not siRNA-mediated gene silencing and that all three proteins bind to the mRNA 5′ m⁷GTP cap-protein complex. Mechanistically, we propose the Ajuba LIM proteins interact with the m⁷GTP cap structure via a specific interaction with eIF4E that prevents 4EBP1 and eIF4G interaction. In addition, these LIM-domain proteins facilitate miRNA-mediated gene silencing by acting as an essential molecular link between the translationally inhibited eIF4E-m⁷GTP-5′cap and Ago1/2 within the miRISC complex attached to the 3′-UTR of mRNA, creating an inhibitory closed-loop complex.