The HIV-1 envelope glycoprotein is expressed on the viral membrane as a trimeric complex, formed by three gp120 surface glycoproteins non-covalently associated with three membrane-anchored gp41 subunits. The labile nature of the association between gp120 and gp41 hinders the expression of soluble, fully cleaved, trimeric gp140 proteins for structural and immunization studies. Disruption of the primary cleavage site within gp160 allows the production of stable gp140 trimers, but cleavage-defective trimers are antigenically dissimilar from their cleaved counterparts. Soluble, stabilized, proteolytically cleaved, trimeric gp140 proteins can be generated by engineering an intermolecular disulfide bond between gp120 and gp41 (SOS), combined with a single residue change, I559P, within gp41 (SOSIP). We have found that SOSIP gp140 proteins based on the subtype A HIV-1 strain KNH1144 form particularly homogenous trimers compared to a prototypic strain (JR-FL, subtype B). We now show that the determinants of this enhanced stability are located in the N-terminal region of KNH11144 gp41 and that, when substituted into heterologous Env sequences (e.g., JR-FL and Ba-L) they have a similarly beneficial effect on trimer stability. The stabilized trimers retain the epitopes for several neutralizing antibodies (b12, 2G12, 2F5 and 4E10) and the CD4-IgG2 molecule, suggesting that the overall antigenic structure of the gp140 protein has not been adversely impaired by the trimer-stabilizing substitutions. The ability to increase the stability of gp140 trimers might be useful for neutralizing antibody-based vaccine strategies based on the use of this type of immunogen.