Функции

ATP + H₂O + vitamin B12(out) = ADP + phosphate + vitamin B12(in)

http://bip.weizmann.ac.il/oca-bin/ocashort?id=1l7v

At the ATP binding sites, cyclotetravanadate molecules are bound to the transporter (ball and stick models at the BtuD interface. Vitamin B12 is delivered to the periplasmic side of the transporter by a binding protein (BtuF, not shown), then translocated through a pathway provided at the interface of the two membrane-spanning BtuC subunits. It finally exits into the cytoplasm at the large gap between the four subunits (arrows). This transport cycle is powered by the hydrolysis of ATP by the ABC cassettes BtuD.

(http://www-ssrl.slac.stanford.edu/research/highlights_archive/Rees_ABC.html)

Each subunit of BtuC has 10 membrane-spanning a-helices. Two short a-helices (labeled, "L1" and "L2") are shown as thick backbone. These helices are proposed to make important interactions with BtuD during gating of the B12 channel. A cluster of three Arg residues on each subunit ("Arg Pocket", above) are thought to provide positively charged "holes" that accept two negatively charged "knobs" on BtuF (vitamin B12-binding protein) when it docks with this complex. See Locher, et al. for details.
During active transport, the vitamin B12-BtuF complex releases the B12 through a gate at the interface between the BtuC subunits (closed in this structure). This allows B12 to enter the cytosol via the large water-filled opening between the BtuC subunits.
BtuD is the ATP-Binding Cassette of this "ABC transporter". The ATP sites in this structure are marked by the "Vanadate" inhibitor* (shown as Spacefill, colored CPK). Conformational changes during ATP hydrolysis at the interacting active sites of BtuD are transmitted to BtuC. This in turn, causes the channel gate to open and allows B12 to enter the cell.

(http://info.bio.cmu.edu/Courses/BiochemMols/Channels/1l7v.htm)

Заключение: белок BtuD выполняет функцию переносчика витамина В12. Молекула витамина присоединяется к определенному месту на внешней поверхности белка (при помощи особого белка переносчика BtuF). Далее молекула проходит вглубь мембраны, проникая между двумя субьединицами, BtuC. Образованное двумя субьединицами пространство со сквозным проходом (которое лежит ниже мембраны, непосредственно внутри клетки) позволяет молекуле витамина проходить в интрацеллюлярное пространство.

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