The catalytic activity of microRNA Argonautes plays a modest role in miRNA star strand cleavage in C. elegans

Many Argonaute proteins in the microRNA (miRNA) pathway have the ability to cleave RNA (slicing) as part of the miRNA-induced silencing complex (miRISC), despite the fact that miRISC-mediated target repression is generally independent of target cleavage. Here we use C. elegans to examine the role of miRISC-mediated slicing in organismal development. We find that unwinding and decay of the miRNA star strand is weakly defective in the absence of slicing, with the largest effect observed in embryos. Argonaute-Like Gene 2 (ALG-2) is more dependent on slicing for unwinding than ALG-1. The miRNAs that displayed the greatest (although still very minor) dependence on slicing for unwinding tend to form stable duplexes with their star strand, and in some cases, lowering duplex stability alleviates dependence of unwinding on slicing. While a small number of miRNA guide strands are reduced in slicing mutants, the basis of this is unclear since changes were not dependent on EBAX-1 (Elongin BC-Binding AXon regulator), a factor in the Target-Directed miRNA Degradation (TDMD) pathway. Expression of mRNAs and gross phenotypic characteristics were wild type in the slicing mutants, indicating that 1) slicing is not required for target regulation and 2) the weak unwinding defect is not sufficient to significantly reduce the pool of functional miRISC. Thus, in contrast to a previous study, miRISC slicing in C. elegans is dispensable in standard laboratory growth conditions.