Editing ABC transporter (section)

=== Transmembrane domain (TMD) ===
Most transporters have transmembrane domains that consist of a total of 12 α-helices with 6 α-helices per monomer. Since TMDs are structurally diverse, some transporters have varying number of helices (between six and eleven). The TM domains are categorized into three distinct sets of folds: ''type I ABC importer'', ''type II ABC importer'' and ''ABC exporter'' folds. The classification of importer folds is based on detailed characterization of the sequences.<ref name=rees/>

The type I ABC importer fold was originally observed in the ModB TM subunit of the [[molybdate]] transporter.<ref name=modb>{{cite journal | vauthors = Hollenstein K, Frei DC, Locher KP | title = Structure of an ABC transporter in complex with its binding protein | journal = Nature | volume = 446 | issue = 7132 | pages = 213–6 | date = Mar 2007 | pmid = 17322901 | doi = 10.1038/nature05626 | bibcode = 2007Natur.446..213H | s2cid = 4417002 }}</ref> This diagnostic fold can also be found in the MalF and MalG TM subunits of MalFGK<sub>2</sub><ref name=malkoldham>{{cite journal | vauthors = Oldham ML, Khare D, Quiocho FA, Davidson AL, Chen J | title = Crystal structure of a catalytic intermediate of the maltose transporter | journal = Nature | volume = 450 | issue = 7169 | pages = 515–21 | date = Nov 2007 | pmid = 18033289 | doi = 10.1038/nature06264 | bibcode = 2007Natur.450..515O | s2cid = 4384771 }}</ref> and the Met transporter MetI.<ref name="pmid18621668">{{cite journal | vauthors = Kadaba NS, Kaiser JT, Johnson E, Lee A, Rees DC | title = The high-affinity E. coli methionine ABC transporter: structure and allosteric regulation | journal = Science | volume = 321 | issue = 5886 | pages = 250–3 | date = Jul 2008 | pmid = 18621668 | pmc = 2527972 | doi = 10.1126/science.1157987 | bibcode = 2008Sci...321..250K }}</ref> In the MetI transporter, a minimal set of 5 transmembrane helices constitute this fold while an additional helix is present for both ModB and MalG. The common organization of the fold is the "up-down" topology of the TM2-5 helices that lines the translocation pathway and the TM1 helix wrapped around the outer, membrane-facing surface and contacts the other TM helices.

The type II ABC importer fold is observed in the twenty TM helix-domain of BtuCD<ref name=btucd/> and in Hi1471,<ref name=hi1471>{{cite journal | vauthors = Pinkett HW, Lee AT, Lum P, Locher KP, Rees DC | title = An inward-facing conformation of a putative metal-chelate-type ABC transporter | journal = Science | volume = 315 | issue = 5810 | pages = 373–7 | date = Jan 2007 | pmid = 17158291 | doi = 10.1126/science.1133488 | s2cid = 10531462 | url = https://authors.library.caltech.edu/51627/7/Pinkett-SOM.pdf }}</ref> a homologous transporter from ''Haemophilus influenzae''. In BtuCD, the packing of the helices is complex. The noticeable pattern is that the TM2 helix is positioned through the center of the subunit where it is surrounded in close proximity by the other helices. Meanwhile, the TM5 and TM10 helices are positioned in the TMD interface. The membrane spanning region of ABC exporters is organized into two "wings" that are composed of helices TM1 and TM2 from one subunit and TM3-6 of the other, in a domain-swapped arrangement. A prominent pattern is that helices TM1-3 are related to TM4-6 by an approximate twofold rotation around an axis in the plane of the membrane.<ref name=rees/>

The exporter fold is originally observed in the Sav1866 structure. It contains 12 TM helices, 6 per monomer.<ref name=rees/>