Bacteriophage lambda has four adjacent genes, S, R, Rz and Rz1, dedicated to host cell lysis. S encodes the holin and antiholin and R encodes the endolysin while Rz1 and Rz are an outer membrane lipoprotein and a type II signal anchor protein called spanins. The Rz-Rz1 complex spans the periplasm ane carries out the final step in the process of host lysis. Endolysin-mediated degradation of the cell wall is a prerequisite for conformational changes in the Rz-Rz1 complex leading to the juxtaposition and fusion of the two membranes. Fusion removes the last physical barrier to efficient release of progeny virions (Berry et al. 2008; Rajaure et al. 2015).

Spanins are phage lysis proteins required to disrupt the outer membrane. Phages employ either two-component spanins or unimolecular spanins in this final step of Gram-negative host lysis. Two-component spanins like Rz-Rz1 from phage lambda consist of an integral inner membrane protein: i-spanin, and an outer membrane lipoprotein: o-spanin, that form a complex spanning the periplasm. Two-component spanins exist in three different genetic architectures; embedded, overlapped and separated. In contrast, the unimolecular spanins, like gp11 from phage T1, have an N-terminal lipoylation signal sequence and a C-terminal transmembrane domain to account for the topology requirements. Young et al. have proposed a model for spanin function for both spanin types. The model follows a common theme of the outer membrane getting fused with the inner membrane, effecting the release of progeny virions. Kongari et al. 2018 described a SpaninDataBase which consists of 528 two-component spanins and 58 unimolecular spanins. Primary sequence analysis revealed significant differences in the secondary structure predictions for the periplasmic domains of the two-component and unimolecular spanin types, as well as within the three different genetic architectures of the two-component spanins. Using a threshold of 40% sequence identity over 40% sequence length, the authors grouped the spanins into 143 i-spanin, 125 o-spanin and 13 u-spanin families. More than 40% of these families from each type were singletons, underlining the extreme diversity of this class of lysis proteins. Multiple sequence alignments of periplasmic domains demonstrated conserved secondary structure patterns and domain organization within family members. Furthermore, analysis of families with members from different architecture allowed interpretation of the evolutionary dynamics of spanin gene arrangement. Also, the potential universal role of intermolecular disulfide bonds in two-component spanin function was substantiated through bioinformatic and genetic approaches. A novel lipobox motif, AWAC, was identified and experimentally verified.  These findings establish that spanins, like viral membrane fusion proteins, adopt different strategies to achieve fusion of the inner and outer membranes (Kongari et al. 2018).

Because of the extensive classification of spanins carried out by Kongari et al. 2018, TCDB does not inculde the large diversity of the proteins found in Nature.  The interested user is referred to Kongari et al. 2018 and their database for comprehensive consideratioins of these proteins. TCDB lists only the spanins that are related to the Rz-Rz1 proteins.