Protein lysine methylation occurs extensively in the Crenarchaeota, a major 
kingdom in the Archaea. However, the enzymes responsible for this type of 
posttranslational modification have not been found. Here we report the 
identification and characterization of the first crenarchaeal protein lysine 
methyltransferase, designated aKMT, from the hyperthermophilic crenarchaeon 
Sulfolobus islandicus. The enzyme was capable of transferring methyl groups to 
selected lysine residues in a substrate protein using S-adenosyl-l-methionine 
(SAM) as the methyl donor. aKMT, a non-SET domain protein, is highly conserved 
among crenarchaea, and distantly related homologs also exist in Bacteria and 
Eukarya. aKMT was active over a wide range of temperatures, from ~25 to 90 °C, 
with an optimal temperature at ~60 to 70 °C. Amino acid residues Y9 and T12 at 
the N terminus appear to be the key residues in the putative active site of 
aKMT, as indicated by sequence conservation and site-directed mutagenesis. 
Although aKMT was identified based on its methylating activity on Cren7, the 
crenarchaeal chromatin protein, it exhibited broad substrate specificity and was 
capable of methylating a number of recombinant Sulfolobus proteins overproduced 
in Escherichia coli. The finding of aKMT will help elucidate mechanisms 
underlining extensive protein lysine methylation and the functional significance 
of posttranslational protein methylation in crenarchaea.