Type I secretion system

 The type I secretion system (T1SS) is one of the mechanisms used by Gram-negative bacteria to secrete proteins to the external medium. Secretion occurs in a single, energy-coupled step via an exporter complex that spans the inner and outer membranes (Fig. 1). The relatively simple exporter complex consists of only three protein subunits: an inner membrane-bound ATP-binding cassette (ABC) protein that forms a complex with a membrane fusion protein (MFP) in the periplasmic space, and an outer membrane protein (OMP) that resides in the periplasmic space and is embedded in the outer membrane.

Figure 1. A model of bacterial T1SS. ABC protein (white), membrane fusion protein (MFP; light gray), and outer membrane protein (OMP; dark gray) form a transporter complex protruding through the inner and outer membranes of Gram-negative bacteria. The secretion signal is shown by a gray box near the C-terminal
end of the passenger protein . Protein secretion occurs in a single step, bypassing the periplasmic space directly to the extracellular medium (the direction of protein secretion is shown by a large shaded arrow). ATP hydrolysis by the ATPase domain of ABC protein provides energy for protein transport. The C-terminal secretion signal is recognized by the ABC protein, stimulating conformational change of the transporter complex and ATP hydrolysis, which leads to secretion of the passenger protein [40]. A β-roll structure, formed by the repetitive sequences in the presence of Ca2+, is also shown.

The ABC protein subunit of the T1SS consists of two transmembrane domains (TMDs) that are embedded in the inner membrane, and two nucleotide-binding domains (NBDs) that are exposed to the cytoplasm. The NBD has ATPase activity and provides the energy needed for protein secretion. The ABC protein of the T1SS belongs to the well-characterized ABC protein superfamily, the largest protein superfamily found in all kingdoms of life, which is related to the import or export of various molecules, from small ions to polysaccharides and proteins.

cited from :Cell. Mol. Life Sci. 63 (2006) 2804–2817