Brown University, Providence, Rhode Island

A microbiologist wonders what turns us on.

An Internet search for the words 'pheromone attractant' pulls up products ranging from human aphrodisiacs to control measures for the Colorado potato beetle.

But sexual chemistry is not only important to humans and beetles, it is also relevant to many fungi. Fungal peptide pheromones are often released by one mating type to attract a partner of the opposite sex, thereby initiating the programme of sexual differentiation. This signalling is often highly specific so that pheromones attract only potential partners and not unwanted suitors.

Work by Joseph Heitman and his colleagues at Duke University in Durham, North Carolina, provides a new spin on pheromone signalling in fungi (Y.-P. Hsueh et al. EMBO J. 28, 1220–1233; 2009). While studying the fungal pathogen Cryptococcus neoformans, the authors became curious about the function of an uncharacterized pheromone-receptor-like gene.

It turns out that this gene, CPR2, encodes a constitutively active receptor that stimulates downstream mating events in both the presence and absence of pheromones. During sexual differentiation, expression of CPR2 is upregulated and supplements the activity of conventional pheromone receptors. A single amino-acid substitution in the Cpr2 protein, in a transmembrane domain that is highly conserved among pheromone receptors, was shown to be responsible for constitutive signalling activity.

This demonstrates that the sexual lifestyles of unicellular organisms can be much more complicated than they first seem. Furthermore, constitutively active receptors have been implicated in many signal-transduction processes in mammalian cells. It remains to be seen whether sexual activity in more complex organisms also involves signalling components that are continuously turned on.