A novel serine/threonine protein kinase from Giardia intestinalis (GiPKB) was isolated by a combination of PCR techniques. Analysis of the GiPKB sequence indicated that the encoded protein appears to be a member of a novel subgroup of serine/threonine protein kinases known as protein kinase B. Reverse transcription PCR and Northern hybridization showed that the transcription of GiPKB is developmentally regulated. The GiPKB was expressed as a recombinant protein, which was characterized and shown to have a protein kinase activity. The preferred substrate for the GiPKB was histone H1, while histone H2A, GSK3 peptide, GS peptide, and Kemptide were phosphorylated at about 96, 73, 5 1, and 40% of the activity with histone H1, respectively. Neither cAMP, Ca2+, nor Ca2+/calmodulin stimulated the enzyme activity. The GiPKB utilized ATP rather than GTP as a phosphate donor with an apparent Km of 20 M. The identification and characterization of this differentially and constitutively expressed GiPKB should allow further analysis of the regulation and signal transduction pathways in Giardia. (c) 2005 Elsevier Inc. All rights reserved.