Viruses have a smart way of protecting their precious RNA - it's held in a protein shell called a capsid until the virus can get into a cell, at which point the RNA is released and hijacks the cell's machinery to make more copies of the virus. It's such a clever mechanism that it would be great to copy for the delivery of gene therapy but unfortunately, viral proteins have a habit of setting off immune responses. So Kenneth Woycechowsky's group at the University of Utah, US, have modified a non-viral protein so that it will carry RNA.
Woycechowsky's group showed that by introducing just four mutations into a small protein that self assembles to form a capsid they could convert the capsid from having a slight negative charge on the internal surface to having a much larger positive charge. As RNA is negatively charged, when the protein was assembled in cells, RNA automatically inserted itself into the capsid. The host-guest system is based purely on charge, although the lengths of RNA inserted seem to be in the 200-350 base range. Being able to understand and control what the capsid carries could help in the development of vehicles for gene therapy.