REGULATION OF DRUG TRANSPORTER ACTIVITY - Drug Transporters: Molecular Characterization and Role in Drug Disposition - page 525

these proteins. Oligomerization can be homomeric (self-association) or heteromeric
(association with a different polypeptide). The heteromeric composition of most pro-
tein complexes gives the cells an additional level of variability and complexity that
it can use for its activity. Often, heteromeric compositions of protein complexes are
tissue specific or developmental specific, and multiple genes can control the activity
of a single heteromeric protein complex.

17.6.1. Homo-oligomerization

In recent years, there have been numerous studies showing that transport proteins
often exist as oligomers. Oligomerization plays critical role in various aspects of
transporter function. Each subunit in the oligomer may form a pore itself and allows
the translocation of its substrate, a mechanism mimicking water channel CHIP28. 30
On the other hand, several subunits in the oligomer may be required to form a sin-
gle pore, as in K + channels. 31 In addition to the functional role mentioned above,
oligomerization is also believed to play a role in the membrane trafficking and sta-
bility of transporters. After synthesis in the endoplasmic reticulum (ER), proteins
undergo a strict process of quality control. Newly synthesized transporters may con-
tain retention signals and are thereby retained in the ER. Oligomerization may shield
or hide such signals and therefore is essential for the egress of transporters from the
ER for subsequent targeting to the plasma membrane. 32 − 34
One example of the homo-oligomerization of drug transporters is the human or-
ganic anion transporter OAT1. 35 Chemical cross-linking of intact membrane proteins
from LLC-PK1 cells stably expressing hOAT1 as well as from rat kidney converted
quantitatively OAT1 monomer to a putative trimer and a higher order of oligomer indi-
cates that OAT1 is present in the membrane as multimeric complexes. The oligomers
are not disulfide bonded, because removing the reducing reagent -mercaptoethanol

from SDS gels did not promote oligomerization of OAT1. When coexpressed in LLC-
PK1 cells, FLAG-tagged hOAT1 coimmunoprecipitated with myc-tagged hOAT1.
The hOAT1 oligomer was also detected in gel filtration chromatography of to-
tal membranes from hOAT1-expressing LLC-PK1 cells. Cell surface biotinylation
with membrane-impermeable reagents and metabolic labeling with [ 35 S]methionine
followed by immunoprecipitation showed that the oligomeric hOAT1 did not contain
any other proteins. Therefore, hOAT1 exists in the plasma membrane of LLC-PK1
cells as a homo-oligomer, possibly trimer, and higher order of oligomer. However,
the functional consequence of such oligomerization remains to be elucidated.

17.6.2. Hetero-oligomerization

Drug transporters are often seen to form hetero-oligomers with their associating pro-
teins. The key organs for drug disposition, such as kidney, brain, intestine, liver, and
placenta, are made of polarized epithelial cells. The capacity of any polarized epithe-
lial cell type to mediate a specific transport process is dependent on its capacity to
deliver the appropriate transport proteins to its apical and basolateral surfaces. The
same transport proteins may be called upon to serve as apical or basolateral proteins in