Mapping Regions oftheMatrix Protein ofVesicular Stomatitis Virus WhichBindtoRibonucleocapsids, Liposomes, and Monoclonal Antibodies

Thematrix (M)protein ofvesicular stomatitis virus (VSV)appearstofunction as a bridge between the ribonucleocapsid (RNP) coreandtheenvelope inassembly ofthevirion. Twosuchproperties wouldnecessitate atleast onesite forinteraction withthenucleocapsid andonewiththeenvelope. Inthis study M protein was found tomediate theinvitro binding toRNPcoresofphospholipid vesicles, representing membrane structures. TheM protein could bindinitially toeither thevesicles ortheRNPcorestopromoteRNP-vesicle association. A trypsin-resistant fragment (MT)ofM protein, missing theinitial 43aminoacids fromitsaminoterminus, reconstituted withacidic phospholipid vesicles withthesame binding efficiency as didwholeM protein, suggesting thatthecarboxy-terminal 81%retained those regions oftheM protein whichinteract witha lipid bilayer. TheMT protein, however, was considerably less efficient thanintact M protein as an inhibitor ofin vitro virus transcription; almost 2.5-fold more MT protein thanintact M protein was required for50% inhibition ofVSVtranscription, indicating that a site forinteraction withtheRNP coremay havebeenlost. A monoclonal antibody whichisable toreversetheinvitro inhibition oftranscription byM protein didnotreact byimmunoblotting withMT protein. Partial tryptic digests oftheM protein probedwiththis monoclonal antibody indicated thatepitope 1lies between aminoacidresidues 18and43.Thisregion appearstobea site that promotes interaction oftheM protein withtheRNPcoreofVSV.Monoclonal antibodies toepitopes 2and 3,whichexhibit some overlap inbinding toM protein butdonotreversetranscription inhibition, weremapped bycleavage withN-chlorosuccinimide atregions ina carboxy direction fromepitope 1.