NMR structure of Streptomyces killer toxin-like protein, SKLP: Further evidence for the wide distribution of single-domain beta gamma-crystallin superfamily proteins

A protein isolated from the culture supernatant of the soil bacterium, Stre ptomyces sp. F-287, exhibits cytocidal effects for both budding and fission yeasts, and causes morphological changes of yeasts and filamentous fungi. This protein, which was the first killer toxin-like protein for yeasts iden tified in the Streptomyces microorganism, was named SKLP (Streptomyces kill er toxin-like protein). Since the amino acid sequence of the protein, as de termined by sequential Edman degradations, seemed to be unique, we determin ed the structure by NMR spectroscopy. Although the actual target of SKLP in yeasts has not been determined yet, the structure might give us a clue to characterize the targets. The solution structure of SKLP determined by NMR, however, turned out to be a single-domain crystallin-like protein, with tw o Greek key motifs and a short extra beta -strand at the N terminus. The fi nal ensemble of 20 NMR structures overlaid onto their mean coordinate with rmsd values of 0.32(+/-0.06) Angstrom for the backbone atoms involved in th e secondary structure elements. As a yeast killer toxin, WmKT, isolated fro m the yeast strain Williopsis mrakii also has a Greek key beta -barrel fold , we have made a detailed comparison of the structural features of SKLP wit h the other crystallin superfamily proteins. It is very interesting that SK LP has a unique electrostatic potential distribution on the molecular surfa ce. Namely, one surface of the beta -barrel fold in SKLP has a large negati vely charged region, with an isolated positive charge of the Arg62 side-cha in at the center. The edge of this surface is surrounded by positively char ged residues, including Arg31, Arg65 and Arg74. The salient features of the charge distribution on this surface and the cluster of Arg residues might be related to the target binding of SKLP. (C) 2001 Academic Press.