An amphipathic helical motif common to tumourolytic polypeptide NK‐lysin and pulmonary surfactant polypeptide SP‐B

The tumour‐lysing and antimicrobial polypeptide NK‐lysin and the pulmonary surfactant‐associated polypeptide SP‐B exhibit 24% residue identities (49% similarities), including six half‐cystine residues in the same disulphide bonding pattern, and similar far‐UV circular dichroism spectra corresponding to 45–55% α‐helix and 20–25% β‐sheet structures. From this, we conclude that the conformations of NK‐lysin and SP‐B are similar. In contrast, the functional properties of the two proteins are dissimilar: SP‐B does not exhibit antibacterial activity and NK‐lysin does not significantly effect phospholipid spreading at an air/water interface. Saposins, which solubilize lipids and activate lysosomal hydrolases, the pore‐forming amoebapores, and parts of acid sphingomyelinase and acyloxyacylhydrolase, also share 18–27% sequence identities with NK‐lysin (and SP‐B), including the six conserved half‐cystine residues. The inclusion of NK‐lysin extends the family of saposin‐like polypeptides, all members of which appear to interact with lipids. Strictly conserved structural features with implications for helix topology and lipid interactions are observed.