Three Multiheme Cytochromes c from the Hyperthermophilic Archaeon Ignicoccus hospitalis: Purification, Properties and Localization.

Three different multiheme cytochromes c were purified from cell extracts of the hyperthermophilic archaeon Ignicoccus hospitalis. One tetraheme cytochrome, locus tag designation Igni_0530, was purified from membrane fractions together with the iron-sulfur protein Igni_0529. Two octaheme cytochromes, Igni_0955 and Igni_1359, were purified from soluble fractions but were also present in the membrane fraction. N-terminal sequencing showed that three of the four proteins had their signal peptides cleaved off, while results were ambigious with Igni_0955. In contrast, mass spectrometry of Igni_0955 and Igni_1359 resulted in single mass peaks including the signal sequences and eight hemes per subunit so that both forms might be present in the cell. Igni_0955 and Igni_1359 belong to the hydroxylamine dehydrogenase family (HAO; 29% mutual identity). HAO or reductase activities with inorganic sulfur compounds were not detected. Igni_0955 was reduced by enriched I. hospitalis hydrogenase at a specific activity of 243 nmol (min•mg hydrogenase)-1 while activity was non-existent with Igni_0530 and low with Igni_1359. Immuno-electron microscopy of ultrathin sections showed that Igni_0955 and Igni_1359 are located in both I. hospitalis membranes and also in the intermembrane compartment. We concluded that these cytochromes might function as electron shuttles between the hydrogenase in the outer cellular membrane and cellular reductases whereas Igni_0530 might be part of the sulfur-reducing machinery