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Vol. 15, Issue 3, 1479-1486, March 2004

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Adenylyl Cyclase G Is Activated by an Intramolecular Osmosensor

School of Life Sciences, University of Dundee, Dundee DD1 5EH, United Kingdom

Submitted August 25, 2003; Revised November 19, 2003; Accepted December 2, 2003
Monitoring Editor: Peter Devreotes

Adenylyl cyclase G (ACG) is activated by high osmolality and mediates inhibition of spore germination by this stress factor. The catalytic domains of all eukaryote cyclases are active as dimers and dimerization often mediates activation. To investigate the role of dimerization in ACG activation, we coexpressed ACG with an ACG construct that lacked the catalytic domain (ACGcat) and was driven by a UV-inducible promoter. After UV induction of ACGcat, cAMP production by ACG was strongly inhibited, but osmostimulation was not reduced. Size fractionation of native ACG showed that dimers were formed between ACG molecules and between ACG and ACGcat. However, high osmolality did not alter the dimer/monomer ratio. This indicates that ACG activity requires dimerization via a region outside the catalytic domain but that dimer formation does not mediate activation by high osmolality. To establish whether ACG required auxiliary sensors for osmostimulation, we expressed ACG cDNA in a yeast adenylyl cyclase null mutant. In yeast, cAMP production by ACG was similarly activated by high osmolality as in Dictyostelium. This strongly suggests that the ACG osmosensor is intramolecular, which would define ACG as the first characterized primary osmosensor in eukaryotes.

^* Corresponding author. E-mail address: p.schaap{at}dundee.ac.uk.

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