Moloney murine leukemia virus genomic RNA packaged in the absence of a full complement of wild type nucleocapsid protein

The current model for MLV genomic RNA (gRNA) packaging predicts that of the thousands of Gag proteins in a budding virion, only a small number (≤1%) may be necessary to recruit gRNA. Here, we examined the threshold limits of functional Gag required to package gRNA using wild-type (WT) and packaging deficient mutant nucleocapsid (NC) phenotypically mixed virions. Although gRNA packaging was severely diminished for the NC mutant, the residual encapsidated RNA dimer displayed motility on gels, thermostability, and integrity that was indistinguishable from that of WT. In phenotypically mixed virions, gRNA encapsidation recovered to within approximately two-fold of WT levels when the amount of WT NC was 5-10% of the total. Our results demonstrate that NC's roles in gRNA dimerization and packaging are genetically separable. Additionally, MLV gRNA packaging does not require 100% WT NC, but the amount of functional NC required is greater than the predicted minimum.

► MLV zinc-knuckle mutant and WT Gags readily co-assemble to form functional mixed virions. ► NC requirements for packaging and genome dimer formation are genetically separable. ► Genomic RNA is efficiently packaged with less than a full complement of WT NC Gag. ► Packaging requires 5-10 fold more WT NC than engages Ψ in purified reactions.