Structural similarities between the development-specific protein S from a gram-negative bacterium, Myxococcus xanthus, and calmodulin.

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Abstract
During differentiation of Myxococcus xanthus, a large amount of protein S is produced and assembled on the surface of the myxospore by a process that specifically requires Ca2+. The gene for protein S has been cloned, and two tandemly repeated homologous genes have been found to be within a short distance of each other in the M. xanthus chromosome. We determined the DNA sequence of 3,692 bp encompassing both genes and deduced the amino acid sequences of the two gene products. The gene 1 (upstream) product and the gene 2 (downstream) product show extensive amino acid sequence homology (88%). However, from their structures, protein S was found to be produced from gene 2, indicating that gene 2 is specifically turned on during differentiation. The structure of protein S shows striking similarities with calmodulin: protein S is composed of four internally homologous domains. In particular, the first and the third domains, consisting of 38 residues each, show a high level of homology (79%), and the second and the fourth domains, consisting of 40 residues each, show homology of 65%. In the first and the third domains, there is a common sequence of nine residues, Glu (or Asp)-Asn-Asn-Thr-Ile-Ser-Ser-Val-Lys, which is highly homologous to one of the proposed Ca2+-binding sequences in bovine brain calmodulin, Asp-Gly-Asn-Gly-Thr-Ile-Thr-Thr-Lys.