Pkn9, a Ser/Thr protein kinase involved in the development of Myxococcus xanthus

The Myxococcus xanthus gene, pkn9, encodes a protein that contains significant homology with eukaryotic Ser/Thr protein kinases. The pkn9 gene was singled out of a previously identified family of kinase genes by amplification techniques that displayed differences in kinase gene expression during selected periods of the M. xanthus life cycle. Pkn9 was constitutively expressed during vegetative growth and upregulated during the aggregation stage of early development. It consists of 589 amino acids, and its N-terminal 394 residues show 38% identity with both Pkn1 and Pkn2 of M. xanthus. This region also shows 29, 25 and 29% identify with myosin light-chain kinase, protein kinase C, and cAMP-dependent protein kinase, respectively. A 22-residue hydrophobic transmembrane domain separates the kinase domain from the 173-residue C-terminal domain that resides on the outside of the inner membrane. The C-terminal domain contains two sets of tandem repeats of 13 and 10 residues which have no known function. When expressed in Escherichia coli under the T7 promoter, Pkn9 was found to be phosphorylated on serine and threonine residues. Disruption of the pkn9 kinase catalytic subdomains I-III by the insertion of a kanamycin-resistance gene resulted in slightly delayed, smaller and more-crowded fruiting bodies, while spore formation was normal. Total deletion of the pkn9 gene caused severely reduced progression through development resulting in light loose mounds that become slightly more compact over time. Development progressed further at the centre than at the edge of the spot, and spore formation was significantly reduced. Two-dimensional gel analysis revealed that both the disruption and the deletion of pkn9 prevented the expression of five membrane proteins (KREP9-1-4). These results suggest that the loss of Pkn9 kinase activity caused altered fruiting-body formation, the absence of the KREP9 proteins in the membrane, and reduced spore production.