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- I believe the E protein holds more functionality than stated. It is certainly involved in attachment, but it also facilitates fusion with the endosome's membrane as a result of the conformational changes it undergoes when in an relatively acidic environment (pH < 6.6)[1,2], causing the nucleocapsid to be released into the cytoplasm--from there the genome can work towards the ER. Reference:
1. Chambers, T. J., & Monath, T. P. (January 1, 2003). The Flaviviruses: Structure, replication, and evolution. p. 27, 65.
2. Allison, SL (May, 2001). “Mutational evidence for an internal fusion peptide in flavivirus envelope protein E.”. Journal of virology (0022-538X), 75 (9), p. 4268.
- Under the prM/M section:
"This immature viral particle buds into the endoplasmic reticulum and eventually travels via the secretory pathway to the golgi apparatus. As the virion passes through the trans-Golgi Network (TGN) it is exposed to low pH. This acidic environment causes a conformational change in the E protein which disassociates it from the prM protein and causes it to form E homodimers."
The effects of acidity in E protein conformational change are seen in the early endosome. Secretory vessicles, peroxisomes, and pretty much anything besides endosomes and lysosomes have a pH of the cytosol or higher--though enzymes within might change this slightly; The virion is not exposed to pH < 6.6 post-uncoating. Also, the prM protein is cleaved along with most of the polyprotein prior to or shortly after exiting the RER[1,2]. It is by Furin (signalase also cleaves AA's near prM, but on other end) that the prM protein gets partially cleaved--this is how the E protein changes conformation during maturation--not by acidity--since the M protein flanks E proteins on the mature virus. Reference:
1. Mackenzie, JM (11/2001). Assembly and maturation of the flavivirus Kunjin virus appear to occur in the rough endoplasmic reticulum and along the secretory pathway, respectively. Journal of virology (0022-538X), 75 (22), p. 10787.
2. Umareddy, I (2007). Dengue virus serotype infection specifies the activation of the unfolded protein response. Virology journal (1743-422X), 4, p. 91.