Chapter 2 : The Ktn Domain and Its Role as a Channel and Transporter Regulator

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In this chapter, the author describes the Ktn domain and its role as a channel and transporter regulator, briefly considers the other bacterial systems that possess Ktn and Ktn-related domains, and, finally presents a more detailed analysis of one's understanding of the structure and operation of the KefC system. The domain can be found as an integral part of the main channel-forming protein (e.g., MthK, Kch, and KefC) as an ancillary extrinsic membrane protein (KtrA, TrkA) and as a protein attached to the membrane by a single transmembrane span (AmhM in ). The protein is one of the shortest members of the family at 434 residues, truncation of the carboxy-terminal domain being responsible for the shorter length and leading to loss of the Ktn domain. Potassium uptake systems in bacteria essentially fall into three categories: K-transporting primary ATPases, such as Kdp; secondary transporters, such as Kup; and more complex systems, e.g., TrkAEH in and KtrAB in . The cation-proton antiports 2 (CPA2) family of proteins was originally defined around the monovalent cation-proton antiports but is now thought to contain channels as well as transporters. The analysis of the crystal structures of K channels has generated great insights into the mechanism of ion selectivity. It seems almost certain that the analysis of the biochemistry of channels such as MthK and KirBac will provide new insights into the gating mechanism.

Citation: Booth I, Edwards M, Gunasekera B, Li C, Miller S. 2005. The Ktn Domain and Its Role as a Channel and Transporter Regulator, p 21-40. In Kubalski A, Martinac B (ed), Bacterial Ion Channels and Their Eukaryotic Homologs. ASM Press, Washington, DC. doi: 10.1128/9781555816452.ch2
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Figure 1

The organization of Ktn-bearing channels and transporters. The structures are shown schematically, with the sizes of the domains approximately to scale according to the numbers of amino acids. Key: dark gray, membrane domain; black, Ktn domain; white or shaded, SAM domains; black line, linker that covalently attaches Ktn to the membrane domain. Note that TrkA has two covalently linked Ktn and SAM domains. The numbers 1 and 2 refer to the first and second domains throughout (see text for details).

Citation: Booth I, Edwards M, Gunasekera B, Li C, Miller S. 2005. The Ktn Domain and Its Role as a Channel and Transporter Regulator, p 21-40. In Kubalski A, Martinac B (ed), Bacterial Ion Channels and Their Eukaryotic Homologs. ASM Press, Washington, DC. doi: 10.1128/9781555816452.ch2
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Image of Figure 2
Figure 2

Structure and mutations in KefC. The figure depicts the hydrophobicity of KefC ( ) and the locations of the different structural features. The positions of mutations that alter the gating of the KefC channel are indicated ( ; unpublished data). Residues indicated above the Ktn and SAM domains inhibit KefC function when mutated and those below the domain increase spontaneous activity. For the linker the single diminished function mutation is shown in brackets. The conserved sequence between helices 8 and 9 is SEYRHALESDIEP, which extends beyond the core HALESDIEP sequence that has been analyzed previously ( ). Arrows indicate the three critical acidic residues that when mutated alter KefC and KefB activity. The position of the putative voltage sensor is indicated (Rxxx).

Citation: Booth I, Edwards M, Gunasekera B, Li C, Miller S. 2005. The Ktn Domain and Its Role as a Channel and Transporter Regulator, p 21-40. In Kubalski A, Martinac B (ed), Bacterial Ion Channels and Their Eukaryotic Homologs. ASM Press, Washington, DC. doi: 10.1128/9781555816452.ch2
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Table 1

Channels and transporters utilizing Ktn domains

Citation: Booth I, Edwards M, Gunasekera B, Li C, Miller S. 2005. The Ktn Domain and Its Role as a Channel and Transporter Regulator, p 21-40. In Kubalski A, Martinac B (ed), Bacterial Ion Channels and Their Eukaryotic Homologs. ASM Press, Washington, DC. doi: 10.1128/9781555816452.ch2

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