Bacillus virus phi29

The Φ29 DNA packaging motor packages the phage genome into the procapsid during viral replication.[6] The Φ29 packaging motor is structurally composed of the procapsid and the connector proteins, which interact with the pRNA, the packaging enzyme (gp16), and the packaging substrate (genomic DNA-gp3).[6] Because the process of genome packaging is energy-intensive, it must be facilitated by an ATP-powered motor that converts chemical energy to mechanical energy through ATP hydrolysis.[6][14] The Φ29 packaging motor is able to generate approximately 57 piconewtons (pN) of force, making it one of the most powerful biomotors studied to date.[6]

The Φ29 pRNA is a highly versatile molecule that can polymerize into dimers, trimers, tetramers, pentamers, and hexamers.[15] Early studies such as Anderson (1990) and Trottier (1998) hypothesized that pRNA formed intermolecular hexamers, but these studies had a solely genetic basis rather than a microscopy based approach.[16][17][18] In the year 2000, a study by Simpson et al. employed cryo-electron microscopy to determine that, in vivo, only a pentamer or smaller polymer could spatially fit in the virus.[18] Ultimately, single isomorphous replacement with anomalous scattering (SIRAS) crystallography was used to determine that the in vivo structure is a tetramer ring.[19] This discovery aligned with what was known about the structural geometry and necessary flexibility of the packaging motor’s three-way junction.[19] When pRNA is in this tetramer ring form, it works as a part of the DNA packaging motor to transport DNA molecules to their destination location within the prohead capsule.[20] Specifically, the functional domains of pRNA bind to the gp16 packaging enzyme and the structural connector molecule to aid in the translocation of DNA through the prohead channel.[6] After DNA packaging is complete, the pRNA dissociates and is degraded.[21]

Versatility in RNA structure and function provides the ability to assemble nanoparticles for nanomedicinal therapeutics.[7] The pRNA in bacteriophage Φ29 can use its three-way junction in order to self-assemble into nanoparticles.[7]

One major challenge of using pRNA-derived nanoparticles is large-scale production, as most industries are currently unequipped to handle industrial pRNA synthesis.[8] This is primarily because RNA nanotechnology is still an emerging field that lacks industrial application and manufacturing optimization of small RNAs.[25]

Φ29’s DNA packaging system, using pRNA, incorporates a motor for the delivery of therapeutic molecules like ribozymes and aptamers.[8] The small size of pRNA-derived nanoparticles also helps to deliver drugs in tight spaces like blood vessels.[8]

The main difficulty in using aptamer-based drug delivery is sourcing unique aptamers and other multimers for specific treatments for diseases that potentially degrade therapeutic multimers and nanoparticles in vivo.[8] Nanoparticles need to be stabilized as delivery mechanisms in order to adapt to microenvironments that may result in loss of therapeutic cargo.[26]

Triple-negative breast cancer (TNBC) is an aggressive form of breast cancer that accounts for ten to fifteen percent of all breast cancer cases.[27] Chemotherapy is the only viable current treatment for TNBC because the loss of target receptors inherent to the disease causes cancer cells to resist therapeutic pharmaceuticals.[9]

The three-way junction in the Φ29 DNA packaging motor can help sensitize TNBC cells to chemotherapy using a siRNA drug delivery mechanism to inhibit TNBC growth and volume.[9] This treatment can also be combined with anti-cancer drugs like Doxorubicin to enhance therapeutic effects.[9]