The mouse mammary tumor virus (MMTV) Pr77 polypeptide is an essential retroviral structural protein without which infectious viral particles cannot be formed. This process requires specific recognition and packaging of dimerized genomic RNA (gRNA) by Gag during virus assembly. Most of the previous work on retroviral assembly has used either the nucleocapsid portion of Gag， or other truncated Gag derivatives—not the natural substrate for virus assembly. In order to understand the molecular mechanism of MMTV gRNA packaging process， we expressed and purified full-length recombinant Pr77-His₆-tag fusion protein from soluble fractions of bacterial cultures. We show that the purified Pr77-His₆-tag protein retained the ability to assemble virus-like particles (VLPs) in vitro with morphologically similar immature intracellular particles. The recombinant proteins (with and without His₆-tag) could both be expressed in prokaryotic and eukaryotic cells and had the ability to form VLPs in vivo. Most importantly， the recombinant Pr77-His₆-tag fusion proteins capable of making VLPs in eukaryotic cells were competent for packaging sub-genomic MMTV RNAs. The successful expression and purification of a biologically active， full-length MMTV Pr77 should lay down the foundation towards performing RNA⁻protein interaction(s)， especially for structure-function studies and towards understanding molecular intricacies during MMTV gRNA packaging and assembly processes.