It is generally accepted that activation of Hog1p in the absence of osmotic stress results in growth inhibitory effects [46]. Previously we reported that the antifungal effects of fludioxonil, iprodione and ambruticin VS3 are dependent on the Ssk1 – Pbs2 – Hog1p branch of the osmotic stress response pathway [25], so that a prerequisite for phosphorylation of Hog1p is the non-phosphorylated form of the response regulator Ssk1p [47]. It was even reported that the

presence of phosphorylated selleck compound Ssk1p prevented the activation of the MAP3K Ssk2p from unphosphorylated Ssk1p [48]. Ssk1p receives phosphate groups indirectly from HKs via the histidine transfer protein Ypd1p. Our results indicate that this phosphorylation is inhibited only in strains which are exposed to osmotic

stress or which express the wild-type CaNIK1 variants and are treated with fungicides. In strains expressing mutated non-functional CaNIK1 phosphorylation of Ssk1 was not inhibited. This conclusion is in agreement with [23] who showed that fludioxonil treatment of S. cerevisiae expressing the group III DhNik1p decreased the phosphate transfer to a response regulator even in the presence of the endogenous, active HK Sln1. Group III HKs are characterized by an amino acid repeat domain with five to six amino acid repeats, in each of which a PS-341 in vitro single HAMP domain was identified previously, but which are now known to comprise concatenated pairs of HAMP domains [25, 32, 33]. The function of these domains is not Baf-A1 mouse yet Go6983 clear, even though involvement in fungicide susceptibility and in osmosensing were suggested [19, 23, 25, 37]. Previous heterologous expression of truncated proteins, in which

several HAMP domains were deleted from group III HKs, i.e. from CaNik1p [25] and DhNik1p from D. hansenii[37], was not reported to result in inhibition of growth of the respective S. cerevisiae transformants. Whereas in the previous reports only selected HAMP domains were deleted, here we deleted all HAMP domains from CaNik1p (CaNik1pΔHAMP) and observed that the synthesis of this truncated protein in the transformed S. cerevisiae strain was associated with severe growth inhibition. This phenotype could be reversed by additional point mutation in the histidine phosphorylation site of the HisKA domain (H510) or by the expression of CaNIK1ΔHAMP in single gene deletion mutants of the response regulator SSK1 or of one of the components of the Hog1 module namely the MAP2K PBS2 and the MAPK HOG1. This proved that the inhibition of growth of the transformant upon expression of CaNIK1ΔHAMP was dependent on the functionality of both the histidine kinase activity of CaNik1p and the functionality of the Ssk1 – Pbs2 – Hog1 branch of the HOG pathway.