A conserved leucine in the cytoplasmic domain of the Semliki Forest virus spike protein is important for budding.

Budding of alphaviruses at the plasma membrane has been shown to depend on specific amino acids of the spike protein and hydrophobic cavities of the nucleocapsid. Here the function of leucine401 in the cytoplasmic tail of the Semliki Forest virus spike protein was studied. When alanine and threonine were substituted for leucine the budding efficiency decreased. When the alanine mutant virus was passaged and sequenced a valine residue at position 401 was found which could partially restore budding proficiency. These results show that leucine401 together with the previously identified tyrosine399 form a motif that is required for budding.