TEM analysis of mutant virus particles. Electron micrographs of mutant and wild type virus particles. Mutants D51N and D51Q showed mostly heterogenous populations of particles with varying size and morphology (panels D51Q and inset in panel D51N). No virus particles with conical core structures were observed with these two mutants. Additionally, a large pool of virus-like structures inside vesicles released from transfected cells were observed in D51N mutant. With the wild type and D51E virions particles representing immature-like viruses are shown (panels D51E and WT). Mature viruses with conical structures were seen only in the wild type control virus. Occasionally, D51E virions resembling the mature wild type morphology but with aberrant core structure was also observed. Bars indicate 100 nm.