The nuclease A inhibitor represents a new variation of the rare PR-1 fold.

Nuclease A (NucA) from Anabaena sp. is a non-specific endonuclease able to degrade single and double-stranded DNA and RNA. The endonucleolytic activity is inhibited by the nuclease A inhibitor (NuiA), which binds to NucA with 1:1 stoichiometry and picomolar affinity. In order to better understand the mechanism of inhibition, the solution structure of NuiA was determined by NMR methods. The fold of NuiA is an alpha-beta-alpha sandwich but standard database searches by DALI and TOP revealed no structural homologies. A visual inspection of alpha-beta-alpha folds in the CATH database revealed similarities to the PR-1-like fold (SCOP nomenclature). The similarities include the ordering of secondary structural elements, a single helix on one face of the alpha-beta-alpha sandwich, and three helices on the other face. However, a major difference is in the IV helix, which in the PR-1 fold is short and perpendicular to the I and III helices, but in NuiA is long and parallel to the I and III helices. Additionally, a strand insertion in the beta-sheet makes the NuiA beta-sheet completely antiparallel in organization. The fast time-scale motions of NuiA, characterized by enhanced flexibility of the extended loop between helices III and IV, also show similarities to P14a, which is a PR-1 fold. We propose that the purpose of the PR-1 fold is to form a stable scaffold to present this extended structure for biological interactions with other proteins. This hypothesis is supported by data that show that when NuiA is bound to NucA significant changes in chemical shift occur in the extended loop between helices III and IV.

Promotion of altered hepatic foci by 2,3,7,8-tetrachlorodibenzo-p-dioxin and 17beta-estradiol in male Sprague-Dawley rats.

The determination of differences in hormonal regulation of tumor promotion-related response to 2,3,7,8-tetrachlorodibenzop-dioxin (TCDD) between males and females may identify factors contributing to the female-specific hepatocarcinogenicity of TCDD in rats. In the current study, diethylnitrosamine-initiated male Sprague-Dawley rats were exposed to TCDD or corn oil vehicle in the presence and absence of 17beta-estradiol (E2), and cell proliferation and development of preneoplastic altered hepatic foci (AHF) were determined. After 30 weeks of exposure, gamma-glutamyltranspeptidase (GGT)-positive AHF and the number of placental glutathione-s-transferase (PGST)-positive AHF were significantly higher in TCDD-treated rats than in control rats. Both the number and volume fraction of GGT-positive AHF were significantly lower in rats cotreated with E2 regardless of TCDD exposure compared with corresponding non-E2-treated groups and were unaffected by TCDD. In contrast, the number of PGST-positive AHF was significantly higher in E2-treated rats in the absence of TCDD treatment. In addition, whereas E2 had no effect on the volume fraction of PGST-positive foci, the levels in rats cotreated with both E2 and TCDD were significantly higher than in controls. No differences were observed in cell proliferation between TCDD-treated and control rats, although cell proliferation was lower in rats exposed to E2 compared with placebo controls. The weaker potency of tumor promotion and lack of induction of cell replication and DNA damage in male rats likely explain the female-specific hepatocarcinogenicity of TCDD in chronic bioassays.