Biosynthesis, maturation, and acid activation of the Semliki Forest virus fusion protein.
The Semliki Forest virus spike protein has a potent membrane fusion activity which is activated in vivo by the low pH of endocytic vacuoles. The spike protein is composed of two transmembrane subunits, E1 and E2, plus E3, a peripheral polypeptide. Acid-induced conformational changes in the E1 or E2 subunits were analyzed by using monoclonal antibodies specific for the acid-treated spike protein. E1 and E2 reacted with the antibodies after treatment of wild-type or mutant virus at the pH of fusion. The E1 conformational change resembled fusion in its requirement for both low pH and cholesterol. Pulse-chase analysis and intracellular pH treatment were then used to determine the ability of the newly synthesized spike to undergo acid-induced conformational changes. p62, the precursor to E2 and E3, was shown to undergo a pH-dependent conformational change similar to that of E2 and was sensitive to acid very soon after biosynthesis. In contrast, a posttranslational maturation event was required for the conversion of E1 to the pH-sensitive form. E1 maturation occurred fairly late in the exocytic pathway, after the virus spike had passed the medial Golgi but before incorporation of the spike into a new virus particle.
Kielian
M
M
Department of Cell Biology, Albert Einstein College of Medicine, Bronx, New York 10461.
Jungerwirth
S
S
Sayad
K U
KU
DeCandido
S
S
D000118:Acetylglucosaminidase; D000818:Animals; D002460:Cell Line; D002472:Cell Transformation, Viral; D005455:Fluorescent Antibody Technique; D006863:Hydrogen-Ion Concentration; D017038:Mannosyl-Glycoprotein Endo-beta-N-Acetylglucosaminidase; D008561:Membrane Fusion; D009154:Mutation; D011487:Protein Conformation; D012672:Semliki forest virus / Q000235:genetics* / Q000378:metabolism; D014760:Viral Fusion Proteins / Q000096:biosynthesis / Q000302:isolation & purification / Q000378:metabolism*
D014760:Viral Fusion Proteins; D000118:Acetylglucosaminidase; D017038:Mannosyl-Glycoprotein Endo-beta-N-Acetylglucosaminidase