Component of the spanin complex that disrupts the host outer membrane and participates in cell lysis during virus exit. The spanin complex conducts the final step in host lysis by disrupting the outer membrane after holin and endolysin have permeabilized the inner membrane and degraded the host peptidoglycans. Host outer membrane disruption is due to local fusion between the inner and outer membrane performed by the spanin complex. Homodimer; disulfide-linked. Interacts (via C-terminus) with the spanin outer lipoprotein subunit (via C-terminus). Part of the spanin complex which spans the entire periplasmic space. The spanin complex is composed of one homodimer of the i-spanin linked by intermolecular disulfide bonds involving two Cys residues and one homodimer of the o-spanin covalently linked by an intermolecular disulfide bond involving one Cys. The coiled coil region is probably involved in host membrane fusion leading to lysis. Belongs to the Lambdavirus i-spanin family.