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GP64, the major envelope glycoprotein of the Autographa californica multicapsid nucleopolyhedrovirus budded virion, is important for host cell receptor binding and mediates low-pH-triggered membrane fusion during entry by endocytosis. Previous transmembrane (TM) domain replacement studies showed that the TM domain serves a critical role in GP64 function. To extend the prior studies and examine specific sequence requirements of the TM domain, we generated a variety of GP64 TM domain mutations. The mutations included 4- to 8-amino-acid deletions, as well as single and multiple point mutations. While most TM domain deletion constructs remained fusion competent, those containing deletions of eight amino acids from the C terminus did not mediate detectable fusion. The addition of a hydrophobic amino acid (A, L, or V) to the C terminus of construct C8 (a construct that contains a TM domain deletion of eight amino acids from the C terminus) restored fusion activity. These data suggest that the membrane fusion function of GP64 is dependent on a critical length of the hydrophobic TM domain. All GP64 proteins with a truncated TM domain mediated detectable virion budding with dramatically lower levels of efficiency than wild-type GP64. The effects of deletions of various lengths and positions in the TM domain were also examined for their effects on viral infectivity. Further analysis of the TM domain by single amino acid substitutions and 3-alanine scanning mutations identified important but not essential amino acid positions. These studies showed that amino acids at positions 485 to 487 and 503 to 505 are important for cell surface expression of GP64, while amino acids at positions 483 to 484 and 494 to 496 are important for virus budding. Overall, our results show that specific features and amino acid sequences, particularly the length of the hydrophobic TM domain, play critical roles in membrane anchoring, membrane fusion, virus budding, and infectivity.

The Autographa californica Multicapsid Nucleopolyhedrovirus GP64 Protein: Analysis of Transmembrane Domain Length and Sequence Requirements