This N-terminal region, HeLo, has a prion-inhibitory effect in cis on its own prion-forming domain (PFD) and in trans on HET-s prion propagation [1]. The domain is found exclusively in the fungal kingdom. Its structure, as it occurs in the HET-s/HET-S proteins, consists of two bundles of alpha-helices that pack into a single globular domain [1]. The domain boundary determined from its structure and from protease-resistance experiments overlaps with the C-terminal prion-forming domain of HET-s (PF11558 [2]. The HeLo domains of HET-s and HET-S are very similar and their few differences (and not the prion-forming domains) determine the compatibility-phenotype of the fungi in which the proteins are expressed. The mechanism of the HeLo domain-function in heterokaryon-incompatibility is still under investigation, however the HeLo domain is found in similar protein architectures as other cell death and apoptosis-inducing domains. The only other HeLo protein to which a function has been associated is LopB from L. maculans [3]. Although its specific role in L. maculans is unknown, LopB- mutants have impaired ability to form lesions on oilseed rape. The HeLo domain is not related to the HET domain (PF06985) which is another domain involved in heterokaryon incompatibility.

1: The mechanism of prion inhibition by HET-S. Greenwald J, Buhtz C, Ritter C, Kwiatkowski W, Choe S, Maddelein ML, Ness F, Cescau S, Soragni A, Leitz D, Saupe SJ, Riek R; Mol Cell. 2010;38:889-899. PMID:20620958

2: Domain organization and structure-function relationship of the HET-s prion protein of Podospora anserina. Balguerie A, Dos Reis S, Ritter C, Chaignepain S, Coulary-Salin B, Forge V, Bathany K, Lascu I, Schmitter JM, Riek R, Saupe SJ; EMBO J. 2003;22:2071-2081. PMID:12727874

3: Comparative analysis of programmed cell death pathways in filamentous fungi. Fedorova ND, Badger JH, Robson GD, Wortman JR, Nierman WC; BMC Genomics. 2005;6:177. PMID:16336669