HIV-1 is neutralized by a class of antibodies that preferentially recognize a site formed on the assembled viral spike. Such quaternary structure-specific antibodies have diverse neutralization breadths, with antibodies PG16 and PG9 able to neutralize 70 to 80% of circulating HIV-1 isolates while antibody 2909 is specific for strain SF162. We show that alteration between a rare lysine and a common N-linked glycan at position 160 of HIV-1 gp120 is primarily responsible for toggling between 2909 and PG16/PG9 neutralization sensitivity. Quaternary structure-specific antibodies appear to target antigenic variants of the same epitope, with neutralization breadth determined by the prevalence of recognized variants among circulating isolates.