Cross references: Adenosine
Adenosine Triphosphate (ATP)
cyclic Adenosine Monophosphate
(cAMP) Guanosine Triphosphate (GTP)
ATP-Binding Cassette Transporter (Wiki)
http://en.wikipedia.org/wiki/ATP-binding_cassette_transporter "ATP-binding cassette transporters (ABC-transporter) are members of a protein superfamily that is one of the largest and most ancient families with representatives in all extant phyla from prokaryotes to humans."
"In
prokaryotes, importers mediate the uptake
of nutrients into the cell. Eukaryotes do not possess any importers."
"ABC transporters are transmembrane proteins that utilize the energy of adenosine triphosphate
(ATP) hydrolysis to carry out certain biological processes including
translocation of various substrates across membranes and
non-transport-related processes such as translation of RNA and DNA
repair."
"The common feature of all ABC transporters is that they consist of two distinct domains, the transmembrane domain (TMD) and the nucleotide-binding domain (NBD). The TMD, also known as membrane-spanning domain (MSD) or integral membrane (IM) domain, consists of alpha helices,
embedded in the membrane bilayer. ... importers
have a high-affinity binding protein (BP) that specifically associates
with the substrate in the periplasm for delivery to the appropriate ABC
transporter. ... It recognizes a variety of substrates
and undergoes conformational changes to transport the substrate across
the membrane."
Structure of an ABC Importer
"ABC transporters utilize the energy of ATP hydrolysis to transport various substrates across cellular membranes. " The substrate-loaded binding
protein acts as the ligand.
Mechanism of Transport for Importers
File:Abc importer.jpg - Wikipedia,
https://en.wikipedia.org/wiki/File:Abc_importer.jpg 
"The resting state of importers is
inward-facing, where the nucleotide binding domain (NBD) dimer
interface is held open by the TMDs and facing outward but occluded from
the cytoplasm. Upon docking of the closed, substrate-loaded binding
protein towards the periplasmic side of the transmembrane domains, ATP
binds and the NBD dimer closes. This switches the resting state of
transporter into an outward-facing conformation, in which the TMDs have
reoriented to receive substrate from the binding protein. After
hydrolysis of ATP, the NBD dimer opens and substrate is released into
the cytoplasm. "
"Both ABC importers and exporters have a common mechanism in
transporting substrates because of the similarities in their
structures."
Structure of an ABC Exporter
 "In most exporters, the N-terminal transmembrane domain and the
C-terminal ABC domains are fused as a single polypeptide chain,
arranged as TMD-NBD-TMD-NBD." " Exporters do not have the binding protein but have an intracellular domain (ICD)
that joins the membrane-spanning helices and the ABC domain. The ICD is
believed to be responsible for communication between the TMD and NBD."
Mechanism of Transport for Exporters
Proposed Mechanism of Transport for ABC Exporters. This model was based on structural and biochemical studies on MsbA.
Note: Click on the picture to enlarge it so that you can read the labels.
Structure, Function, and Evolution of Bacterial ATP-Binding Cassette Systems (PubMed)
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2415747/?tool=pubmed Full length article. "ABC systems are widespread among living organisms and have been
detected in all genera of the three kingdoms of life, with remarkable
conservation in the primary sequence of the cassette and in the
organization of the constitutive domains or subunits"
This is a discussion of the function of ABC-transporters in prokaryotes.
"ATP-binding cassette transporters (ABC-transporter) are divided into three main functional categories.
In prokaryotes, importers mediate the uptake of nutrients into the cell. The substrates that can be transported include ions, amino acids, peptides, sugars, and other molecules that are mostly hydrophilic. The membrane-spanning region of the ABC transporter protects hydrophilic substrates from the lipids of the membrane bilayer thus providing a pathway across the cell membrane. Eukaryotes do not possess any importers.
Exporters, which are both present in prokaryotes and eukaryotes, function as pumps that extrude toxins and drugs out of the cell. In gram-negative bacteria, exporters transport lipids and some polysaccharides from the cytoplasm to the periplasm.
The third subgroup of ABC proteins do not function as transporters, but
are rather involved in translation and DNA repair processes."
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