PDB ID: 1PRR
Medline Link
PDB file
Greek Key motif
Biology
M. xanthus is a gram-negative, spore-forming soil bacterium. When the colony starves, cells aggregate to form moulds. During this period, some cells differentiate into myxospores while most other cells lyse and die.Protein S is a developmentally regulated extracellular protein, produced by M. xanthus. The organism begins the production of protein S early in its cell cycle, and reaches the peak production of protein S late in its cell cycle. Most protein S is found on the surface of the myxospores.
Protein S does not have a signal sequence that will direct its secretion, thus it has been hypothesized that protein S is released into extracellular space by cell lysis. Upon being released, protein S forms a multi-layer spore coat in presence of Ca2+.
Protein S is a heat-stable, acidic protein. It can bind two Ca2+
ions. N-terminal calcium-binding site has a higher affinity than the C-terminal
calcium-binding site.
Structure
Overall, the structure of protein S is similar to beta-gamma-crystallins -- principal components of the vertebrate eye lens. Protein S contains two domains, and the overall folds of the two domains are essentially identical.Within each domain, there are two Greek key motifs (anti-parallel beta sheets linked with random coil). One motif contains an alpha helix while the other does not. The presence of alpha-helix differentiates protein S from beta-gamma-crystallins. The two motifs come together form two beta sheets in each motif. Each sheet is made of beta-strands a,b and d from one Greek key motif and beta-strand c from the other motif (beta strands are named a, b, c and d from the N-terminus).
Kinks are present b4 and d4 (second and third
beta strand of forth Greek key domain) due to proline residues at positions
152 and 158. These kinks may be responsible for lower stability of the
C-terminal domain and lower affinity of the C-terminal domain for a calcium
ion.