The archaeon Sulfolobus solfataricus contains a membrane-associated protein kinase activity that preferentially phosphorylates threonine residues in vitro

The extreme acidothermophilic archaeon Sulfolobus solfataricus harbors a me mbrane-associated protein kinase activity, Its solubilization and stabiliza tion required detergents, suggesting that this activity resides within an i ntegral membrane protein. The archaeal protein kinase utilized purine nucle otides as phosphoryl donors in vitro. A noticeable preference for nucleotid e triphosphates over nucleotide diphosphates and for adenyl nucleotides ove r the corresponding guanyl ones was observed, The molecular mass of the sol ubilized, partially purified enzyme was estimated to be approximate to 125 kDa by gel filtration chromatography. Catalytic activity resided in a polyp eptide with an apparent molecular mass of approximate to 67 kDa by sodium d odecyl sulfate-polyacrylamide gel electrophoresis, Challenges with several exogenous substrates revealed the protein kinase to be relatively selective . Only casein, histone H4, reduced carboxyamidomethylated and maleylated ly sozyme, and a peptide modeled after myosin light chains (KKRAARATSNVFA) wer e phosphorylated to appreciable levels in vitro. All of the aforementioned substrates were phosphorylated on threonine residues, while histone H4 was phosphorylated on serine as well, Substitution of serine for the phosphoacc eptor threonine in the myosin light chain peptide produced a noticeably inf erior substrate. The protein kinase underwent autophosphorylation on threon ine and was relatively insensitive to a set of known inhibitors of "eukaryo tic" protein kinases.