Comparative Structural Analysis of The Homeodomain Protein in the Bipolar Basidiomycete, Mycoleptodonoides aitchisonii (Bunaharitake)

Mycoleptodonoides aitchisonii is a bipolar mushroom with monokaryons can form clamp cells, regardless of the mating type. Previously, only HD2-18 was identifi ed and characterized at the A locus of Maspi18 (A1). In this study, unlike the A locus in strain Maspi18, HD1-7-like gene (Mahd1-7) and HD2-7 gene (Mahd2-7) were identifi ed at the A locus of the compatible strain Maspi7 (A2), but the nine fl anking genes were highly conserved, suggesting that recombination events may have occurred between HD1 and HD2 genes over evolutionary time. Following phylogenetic and HD protein structural comparisons with other basidiomycetes, homeodomains were identifi ed in HD2-7 and HD2-18, but the former lacked nuclear localization signals (NLS) whereas the latter, like the HD1-7-like protein, did not. These fi ndings indicated that one homeodomain and several NLS on the same or different HD proteins is the minimum suffi cient for driving clamp cell formation in M. aitchisonii. The fi ndings implied that over evolutionary time, unlike HD2, the homeodomain in HD1 does not play a major role in clamp cell formation, and that HD1-7 performs the function of carrying the NLS. Herein, we propose a new hypothesis for clamp cell formation in M. aitchisonii. The fi ndings of this study provide a foundation for understanding the role of heterokaryonization in basidiomycete mushrooms.