Transmembrane topology of the AbsA1 sensor kinase of Streptomyces coelicolor

The sensor kinase AbsA1 (SCO3225) phosphorylates the response regulator AbsA2 (SCO3226) and dephosphorylates AbsA2 approximately P. The phosphorylated response regulator represses antibiotic biosynthesis operons in Streptomyces coelicolor. AbsA1 was predicted to have an atypical transmembrane topology, and the location of its signal-sensing domain is not readily obvious. To better understand this protein and to gain insight into its signal response mechanism, we determined its transmembrane topology using fusions of absA1 to egfp, which is believed to be the first application of this approach to transmembrane topology in the actinomycetes. Our results are in agreement with the in silico topological predictions and demonstrate that AbsA1 has five transmembrane domains, four near the N terminus and one near the C terminus. Unlike most sensor kinases, the largest extracellular portion of AbsA1 is at the C terminus.