Lentiviruses are widespread in a variety of vertebrates, often associated with chronic disease states. However, until the recent discovery of the prehistoric endogenous lentiviruses in rabbits (RELIK) and lemurs (PSIV), it was thought that lentiviruses had no capacity for germline integration and were only spread horizontally in an exogenous fashion. The existence of RELIK and PSIV refuted these ideas, revealing lentiviruses to be present in a range of mammals, capable of germline integration, and far more ancient than previously thought. Using Gag sequences reconstructed from the remnants of these prehistoric lentiviruses, we have produced chimeric lentiviruses capable of infecting nondividing cells and determined structures of capsid domains from PSIV and RELIK. We show that the structures from these diverse viruses are highly similar, containing features found in modern-day lentiviruses, including a functional cyclophilin-binding loop. Together, these data provide evidence for an ancient capsid-cyclophilin interaction preserved throughout lentiviral evolution.

Defining the RNA target selectivity of the proteins regulating mRNA metabolism is a key issue in RNA biology. Here we present a novel use of principal component analysis (PCA) to extract the RNA sequence preference of RNA binding proteins. We show that PCA can be used to compare the changes in the nuclear magnetic resonance (NMR) spectrum of a protein upon binding a set of quasi-degenerate RNAs and define the nucleobase specificity. We couple this application of PCA to an automated NMR spectra recording and processing protocol and obtain an unbiased and high-throughput NMR method for the analysis of nucleobase preference in protein–RNA interactions. We test the method on the RNA binding domains of three important regulators of RNA metabolism.

The HML2 (HERV-K) group constitutes the most recently acquired family of human endogenous retroviruses, with many proviruses less than one million years old. Many maintain intact open reading frames and provirus expression together with HML2 particle formation are observed in early stage human embryo development and are associated with pluripotency as well as inflammatory disease, cancers and HIV-1 infection. Here, we reconstruct the core structural protein (CA) of an HML2 retrovirus, assemble particles in vitro and employ single particle cryogenic electron microscopy (cryo-EM) to determine structures of four classes of CA Fullerene shell assemblies. These icosahedral and capsular assemblies reveal at high-resolution the molecular interactions that allow CA to form both pentamers and hexamers and show how invariant pentamers and structurally plastic hexamers associate to form the unique polyhedral structures found in retroviral cores.