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RTX toxin
The RTX toxin family is a group of cytotoxins produced by Gram-negative bacteria. There are over 1000 known members with a variety of functions. The RTX family is defined by two common features: characteristic repeats in the toxin protein sequences, and extracellular secretion by the type I secretion system (T1SS). The name RTX (repeats in toxin) refers to the glycine and aspartate-rich repeats located at the C-terminus of the toxin proteins, which facilitate export by a dedicated T1SS encoded within the ''rtx'' operon.〔
==Structure and function==
RTX proteins range from 40 to over 600 kDa in size and all contain C-terminally located glycine and aspartate-rich repeat sequences of nine amino acids. The repeats contain the common sequence structure , (where X represents any amino acid), but the number of repeats varies within RTX protein family members. These consensus regions function as sites for Ca2+ binding, which facilitate folding of the RTX protein following export via an ATP-mediated type 1 secretion system (T1SS). Most of the T1SS proteins are encoded within the ''rtx'' operon. The T1SS proteins form a continuous channel spanning both the inner membrane (IM) and outer membrane (OM) of the bacterial cell, preventing RTX toxin exposure to the periplasmic space (between the IM and OM). Type 1 secretion systems components include: an ABC transporter, a membrane fusion protein (MFP), and an outer membrane protein (OMP). The OMP is often encoded outside of the rtx operon as it may have multiple functions within the cell. In ''Escherichia coli'', ''Pasteurella'' ''haemolytica'', and ''Vibrio cholerae'', TolC functions as the OMP in T1SS RTX toxin export. In each case, the ''tolC'' gene is located outside the ''rtx'' operon and encodes a conserved multifunctional protein. During transport, the T1SS recognizes the C-terminal repeats of the RTX toxin, and the C-terminus is transferred first through the channel.〔
The general ''rtx'' gene cluster encodes three protein types: the RTX toxin, an RTX activating acyltransferase, and T1SS proteins. The toxin is inactive until post-translational modification by the cis-encoded RTX toxin activator, which typically occurs within the target cell. The RTX-activating acyltransferase catalyzes the attachment of acyl-linked fatty acids to internally located lysine residues within the RTX toxin. This modification is required in all RTX toxins; however, its exact function in RTX toxicity is not understood. Members of the RTX toxin family display a large range of functions, and typically multiple functional domains.〔 Pore-formation is the only known shared function in RTX cytotoxins, and pores are typically cation-selective allowing for an influx of Ca2+ in target cells.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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