Synthesis andassembly ofmembraneglycoproteins: Presence of leader peptide innonglycosylated precursor ofmembrane glycoprotein ofvesicular stomatitis virus

Translation ofmRNAencoding vesicular sto- matitis virus envelope glycoprotein Gbyamembrane-free ri- bosomal extract obtained fromHeLacells yielded anongly- cosylated protein G1(Mr63,000). Inthepresence ofadded mi- crosomal -membranes, G1wasconverted totheglycosylated protein G2(Mr67,000) which isinserted inthemembrane ves- icles asatransmembrane protein. Labeling withmethionine donated bywheat germinitiator tRNAIet showed that GIbut notG2contains methionine intheNH2-terminal position. De- termination oftheNH2-terminal sequence ofG1,G2,andG showed that aleader peptide of16amino acids ispresent inG1 butabsent from theglycosylated proteins G2andG.This leader peptide contains atleast 62%hydrophobic amino acids andis removed presumably during insertion ofG1into themem- brane. Animportant aspect ofmembrane biogenesis isthemechanism bywhich integral membrane proteins aretransported from the site ofsynthesis andareassembled inthemembrane. Theen- veloped viruses that serve asmodels tostudy membrane structure andfunction also areexcellent systems forstudy-of membrane biogenesis (1). Astudy ofthemechanisms ofsyn- thesis andglycosylation oftheviral membrane glycoproteins andoftheir insertion into themembrane, transport along cel- lular membrane systems, andultimate migration totheplasma membrane oftheinfected cell canhelp toelucidate these- quence ofevents involved inmembrane biogenesis. Because cell surface glycoprotein molecules arebelieved tobeinvolved incellular recognition andinformation exchange processes occurring onthecell membrane (2), insight into these processes maybegained fromstudies onmembrane glycoprotein syn- thesis andassembly. Studies fromthis (3, 4)andother labora- tories (5, 6)have shown that translation invitro ofthe. mRNA coding forthemembrane glycoprotein G(Mr69,000) ofthe enveloped vesicular stomatitis virus (VSV) intheabsence of microsomal membranes results inthesynthesis ofthenongly- cosylated protein G1(Mr63,000). Inthepresence ofadded microsomal membrane vesicles, however, G1isconverted to theglycosylated protein G2(Mr67,000) which isinserted into thevesicle membrane. Itwasalso shown that insertion into the membrane andglycosylation arecotranslational events. Unlike secretory proteins, which arealso synthesized bymembrane- boundribosomes andaredischarged vectorially across the microsomal membrane (7), glycoprotein G2isnotcompletely discharged butspans themembrane (4-6). Ithasbeen postulated that thetransport ofsecretory proteins across membranes isinitiated bytheassociation ofasignal or leader sequence attheNH2terminus ofthenascent polypeptide chain with theendoplasmic reticulum (8, 9). Theformation of