PROSITE documentation PDOC51500

Sin domain profile

Spore formation is an extreme response of some bacteria to adversity. In Bacillus subtilis the proteins of the sin, sporulation inhibition, region form a component of an elaborate molecular circuitry that regulates the commitment to sporulation. SinR (formerly known as Sin) is a tetrameric repressor protein that binds to the promoters of genes essential for entry into sporulation and prevents their transcription. This repression is overcome through the activity of SinI, which disrupts the SinRtetramer through the formation of a SinI-SinR heterodimer. In this complex, SinR is unable to bind to DNA and its repressive effects on transcription are relieved. SinR and SinI share a multimerisation domain of ~40 amino acids [1,2].

The structure reveals an α-helical assembly comprising two domains of approximately equal size, an oligomerisation domain stabilised by an unusual intermolecular hydrophobic core, and a DNA-binding domain formed by the N-terminal residues of SinR, which form five α-helices (see <PDB:1B0N>). The oligomerisation domain is formed by four α-helices, two from the C-terminal residues of SinR and two from the central residues of SinI. The pairs of helices from the respective molecules interpenetrate like the forefingers and thumbs in a handshake. This arrangement vuries an unusually extensive and closely packed intermolecular apolar core, reminiscent of a folded protein domain. The pairs of helices from the respective proteins can be closely superimposed so that conserved hydrophobic residues align at corresponding positions on the inner faces of the helices. This conservation of structure and sequence suggests that the C-terminal helices of two SinR molecules could also interpenetrate in a similar manner to form SinR dimers and tetramers. The interactions between pairs of SinR monomers must be weaker than those in the SinI-SinR heterodimer, since SinI efficiently disrupts SinR tetramers [1,2].

The profile we developed covers the entire Sin domain.