A LargeDeletion intheMatrix DomainoftheHuman Immunodeficiency Virus gag GeneRedirects VirusParticle Assembly fromthePlasmaMembranetothe Endoplasmic Reticulum

Morphogenesis ofretroviruses involves assembly ofthestructural GagandGag-Pol polyproteins with subsequent budding ofthevirus particle fromtheplasma membraneandproteolytic cleavage bytheviral proteinase. Thematrix (MA)domain, representing theN-terminal segment ofGag,plays acritical role inthis process.We constructed an in-frame deletion intheMA coding region (lacking codons 16to99)ofthehuman immunodeficiency virus (HIV) type 1 gaggene.Following transient transfection ofthecomplete proviral DNA carrying thedeletion, themutantpolyprotein was synthesized andproteolytically processed like thewild-type polyprotein. However, release ofvirus particles was reduced approximately 10-fold. Theextracellular particles that were released didnotcontain viral glycoproteins andwere noninfectious. Electron micrographs revealed budding ofvirus particles intotheendoplasmic reticulum (ER)oftransfected cells andlarge numbersof particles within theER.Theseparticles were allimmature andmorphologically indistinguishable from intracisternal A-type particles, a class ofmurine endogenous retrovirus elements. Budding structures atthe plasma membrane were rarelyseenandonlya fewextracellular particles were observed, butincontrast to those intheER,these particles hadthemorphology ofmatureparticles, similar tothat ofwild-type HIV,except forthelackofsurface projections. Morphogenesis ofinfectious retrovirus particles requires themorphopoetic function ofviral coreproteins (encoded by thegag gene) as wellas theincorporation offunctional replication enzymes (proteinase [PR], reversetranscriptase [RT], andintegrase [IN]; derived fromthepolgene) andthe envelope glycoproteins gpl20(SU)andgp41(TM;encoded bytheenv gene;fora reviewofnomenclature ofretroviral proteins, seereference 21). Theproducts ofthegag andpol genes are translated as two polyproteins (Pr55a&and Pr1609agP9I forhumanimmunodeficiency virus type1[HIV1])whicharecoterminal intheir N-terminal segments. Viral assembly isbelieved tooccurbyassociation ofuncleaved polyproteins toformthespherical immature core either in thecytoplasm (type BandD oncoviruses andspumaviruses) orattheplasma membrane(type C oncoviruses andlentiviruses [e.g., HIV]) oftheinfected cell(forreviews, see references 9and41).Ultimately, however, either thepreformed coresor unassembled polyproteins mustmigrate to theplasma membranetoinitiate thebudding andrelease of theimmature virion. Themechanism ofmembranetargeting iscurrently notknowninany retrovirus system,butN-terminalmyristoylation oftheGagandGag-Pol polyproteins, whichisfoundinmost,butnotall, retroviruses hasbeen showntoplay arole inthis process.Intheir detailed analysis ofMason-Pfizer monkeyvirusmorphogenesis, Rheeand Huntershowedthat transport ofthestructural polyproteins isan active andspecific intracellular targeting process and thatadditional signals, besides N-terminal myristoylation, arerequired formembranetargeting (34, 35,37). Asafinal stepinmorphogenesis, thecondensation oftheimmature