Component of the spanin complex that disrupts the host outer membrane and participates in cell lysis during virus exit (PubMed:22904283, PubMed:23387988, PubMed:25870259). 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 (PubMed:25870259). Host outer membrane disruption is due to local fusion between the inner and outer membrane performed by the spanin complex (PubMed:25870259). Homodimer; disulfide-linked (PubMed:23387988). Interacts (via C-terminus) with the spanin outer lipoprotein subunit (via C-terminus) (PubMed:20734329). Part of the spanin complex which spans the entire periplasmic space (PubMed:23387988). 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 (PubMed:23387988). The coiled coil region is probably involved in host membrane fusion leading to lysis. Belongs to the Lambdavirus i-spanin family.