Nucleic acid transport through cell membranes via SID-1

RNA interference (RNAi) is a sequence-specific post-transcriptional gene silencing phenomenon triggered by double-stranded RNA (dsRNA) homologous to target mRNA. In the nematode Caenorhabditis elegans, RNAi is systemic; that is, the gene specific silencing spreads from the site of dsRNA introduction to silence the targeted gene throughout the organism. SID-1 is a transmembrane protein specifically required for systemic RNAi. SID-1 is conserved with homologs present in many sequenced animal genomes including nematodes, most vertebrates, and select other invertebrates, but notably not in Drosophila which demonstrably lacks efficient systemic RNAi. C. elegans SID-1 expressed in Drosophila S2 cells forms a passive channel or pore at the cell surface that allows energy-independent transport of double-stranded RNA (dsRNA) through cell membranes. Work presented here addresses aspects of SID-1 substrate specificity and the mechanism of dsRNA transport. SID-1 likely forms an oligomeric channel that enables specific transport of double-stranded nucleic acids. dsRNA uptake kinetics are consistent with diffusion-mediated transport, and long dsRNA is strongly favored over double-stranded DNA and short dsRNA. Implications for the use of SID-1 in developing RNAi-based therapeutics and large scale RNAi screens are discussed.