ABC motif

Transporters for monocarboxylic acids include (1) the bacterial lactate permease LctP family, (2) eukaryotic proton-linked monocarboxylate transporter MCT family, and (3) monocarboxylate permease. These transporters contain no ATP binding cassette (ABC) motifs and are considered to transport monocarboxylic acids via a proton-coupled reaction (Nakano, Fukaya, and Horinouchi, 2006). 

ABC transporter proteins are a ubiquitous family of proteins characterised by an ATP-binding cassette (ABC) motif (Chapter 42). These ATP-binding proteins belong to one of the largest families known to medical science. The bound ATP is hydrolysed in a process coupled to transport of their substrate. One such protein is the cholesterol transporter known as ABC-Al (not shown in Fig. 41.1). It is found in many tissues where its function is to transfer excess cholesterol to HDL particles. The HDL particles proceed to the liver for disposal. 

The NBD unit harbors several conserved sequence motifs (Walker A and B, ABC-signature motifs), which... 

The ABC transporters are products of one of the largest gene superfamilies. Each consists of two cytoplasmic nucleotide-binding domains (NBDs) and two transmembrane domains (TMDs). The NBDs are highly conserved across the ABC family and contain motifs typical of ATP-binding sites, whereas the TMD structures vary, probably because they are adapted to the wide variety of substrates. In eukaryotes the C-terminal of each NBD is linked to a TMD. In some cases the functional unit is (NBD-TMD)2 and, in others, the first TMD is covalently linked to the second NBD. 

The characteristic Walker A and Walker B motifs that are involved in ATP binding [144] are always found in the ATPase or ABC domains. In addition, a signature motif, also called the LSGGQ motif, is typical of all bacterial ABC domains involved in binding-protein-dependent import. The signature motif is absent in other types of ATPases. 

Fetsch, E. E. and Davidson, A. L. (2002). Vanadate-catalyzed photocleavage of the signature motif of an ATP-binding cassette (ABC) transporter, Proc. Natl Acad. Sci. USA, 99, 9685-9690. 

All ABC proteins contain at least three characteristic peptide sequences the Walker A and B motifs, and the so-called ABC-signature sequence. Whereas the Walker motifs are present in several classes of ATP-binding pro-... 

Sister chromatid cohesion is primarily established by the cohesin complex (23). The cohesin complex is a multiprotein complex that contains the structural maintenance of chromosomes (SMC) family of proteins (24). SMC proteins contain the Walker A motif at their N-termini and the Walker B motif at their C-termini (Fig. 2a). These motifs are brought together by an intramolecular coiled-coil domain to form a functional ATPase domain, which is similar to other ATP-binding cassette (ABC) ATPases, such as RAD50 (25). SMC proteins... 

ABC transporters translocate many different allocrites (transport substrates), but the primary structures of the NBDs show --25% sequence identity across the whole superfamily [9[. Marked sequence conservation is observed over five short regions found in the NBDs (i) the Walker A and (ii) Walker B regions, which are separated by approximately 90-120 amino acids and between which lie the (hi) signature motif [2, 7, 9, 35, 36] and (iv) the glutamine loop (Q-loop) [37[. The most C-terminal motif is the histidine loop (H-loop) [2[. The signature, Q-loop, and H-loop seem to be specific to ABC transporters [9], and the function of these is described in more detail below. A recent description of the importance of a highly conserved aromatic residue has led to the naming of an A-loop at the N-terminus of the NBD [38[. 

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