Preparation and characterization of the extracellular domain of human Sid-1

PLoS One. 2012;7(4):e33607. doi: 10.1371/journal.pone.0033607. Epub 2012 Apr 11.


In C. elegans, the cell surface protein Sid-1 imports extracellular dsRNA into the cytosol of most non-neuronal cells, enabling systemic spread of RNA interference (RNAi) throughout the worm. Sid-1 homologs are found in many other animals, although for most a function for the protein has not yet been established. Sid-1 proteins are composed of an N-terminal extracellular domain (ECD) followed by 9-12 predicted transmembrane regions. We developed a baculovirus system to express and purify the ECD of the human Sid-1 protein SidT1. Recombinant SidT1 ECD is glycosylated and spontaneously assembles into a stable and discrete tetrameric structure. Electron microscopy (EM) and small angle x-ray scattering (SAXS) studies reveal that the SidT1 ECD tetramer is a compact, puck-shaped globular particle, which we hypothesize may control access of dsRNA to the transmembrane pore. These characterizations provide inroads towards understanding the mechanism of this unique RNA transport system from structural prospective.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Animals
  • Baculoviridae / genetics
  • Extracellular Space / metabolism*
  • Glycosylation
  • Humans
  • Membrane Transport Proteins / chemistry*
  • Membrane Transport Proteins / genetics
  • Membrane Transport Proteins / isolation & purification
  • Membrane Transport Proteins / metabolism*
  • Models, Molecular
  • Protein Folding
  • Protein Multimerization
  • Protein Processing, Post-Translational
  • Protein Structure, Quaternary
  • Protein Structure, Tertiary
  • Sequence Homology, Amino Acid
  • Solubility
  • Spodoptera / cytology


  • Membrane Transport Proteins
  • systemic RNA Interference-defective-1 transmembrane family member 1, human