THE PLANT CELL, Vol 6, Issue 10 1467-1475, Copyright © 1994 by American Society of Plant Biologists
Two Classes of Homeodomain Proteins Specify the Multiple A Mating Types of the Mushroom Coprinus cinereus
U. Kues, R. N. Asante-Owusu, E. S. Mutasa, A. M. Tymon, E. H. Pardo, S. F. O'Shea, B. Gottgens and L. A. Casselton
Department of Plant Sciences, University of Oxford, South Parks Road, Oxford, OX1 3RB, United Kingdom
The A mating type locus of the mushroom Coprinus cinereus regulates
essential steps in sexual development. The locus is complex and contains
several functionally redundant, multiallelic genes that encode putative
transcription factors. Here, we compare four genes from an A locus
designated A42. Overall, the DNA sequences are very different (~50%
homology), but two classes of genes can be distinguished on the basis of a
conserved homeodomain motif in their predicted proteins (HD1 and HD2).
Development is postulated to be triggered by an HD1 and an HD2 gene from
different A loci. Thus, proteins encoded by genes of the same locus must be
distinguished from those encoded by another locus. Individual proteins of
both classes recognize each other using the region N-terminal to the
homeodomain. These N-terminal specificity regions (COP1 and COP2) are
predicted to be helical and are potential dimerization interfaces. The
amino acid composition of the C-terminal regions of HD1 proteins suggests a
role in activation, and gene truncations indicate that this region is
essential for function in vivo. A corresponding C-terminal region in HD2
proteins can be dispensed with in vivo. We will discuss these predicted
structural features of the C. cinereus A proteins, their proposed
interactions following a compatible cell fusion, and their similarities to
the a1 and [alpha]2 mating type proteins of the yeast Saccharomyces
cerevisiae.
