1021-4437/04/5101- © 2004
Russian Journal of Plant Physiology, Vol. 51, No. 1, 2004, pp. 127–137. Translated from Fiziologiya Rastenii, Vol. 51, No. 1, 2004, pp. 142–152.
Original Russian Text Copyright © 2004 by Shapiguzov.
Multicellular organisms have developed specialized
tissues with low hindrance to water ﬂows. Nonetheless,
the transfer of water molecules across the cell mem-
brane is the main step in water transport .
The ability of cells to control outward or inward
movement of water and solutes is a matter of principal
signiﬁcance. Cell membranes represent selective com-
plex ﬁlters that regulate the transport of ions, organic
substances, and water. The current knowledge of mem-
brane structure and functions is rapidly expanding .
A number of transmembrane carrier proteins have been
discovered and characterized. However, despite this
progress, molecular basis of the transmembrane water
transport remained poorly understood until recently.
The water permeability of biological membranes was
commonly attributed to diffusion of water across the
lipid bilayer. However, some physiological processes
are associated with translocation of large amounts of
water or with rapid changes in membrane permeability
to water. Such phenomena cannot be explained by
water diffusion across the lipids, which implicates the
existence of water-carrying proteins.
The ﬁrst of such carriers was revealed in the mam-
malian erythrocytes whose cell membrane is highly
permeable to water . The initial observation that
and organic–mercurial substances inhibit water
transport implied the involvement of protein in this pro-
cess . A speciﬁc polypeptide was isolated later and
termed CHIP28 (channel-like integral protein of 28 kD)
[5, 6]. Several homological proteins were revealed in
other mammalian tissues. One of such proteins, called
MIP (major intrinsic protein of lens), had been known
for a long time although its function was unclear .
Later studies demonstrated the presence and, in some
cases, abundance of homological proteins in many
organisms , and their transport activity was proven.
A term “aquaporins” (Aqp) was adopted, and CHIP28
was renamed as Aqp1 . An alternative term—MIP
family—is sometimes used for aquaporins in the litera-
Presently, the number of discovered aquaporins
exceeds two hundred, and the plant aquaporins consti-
tute a considerable part of this family . For exam-
genome contains 35 aqua-
porin genes [11–13], and
contains more than
30 such genes .
Water transport is extremely difﬁcult for quantita-
tive assessment. Unlike the transmembrane ion trans-
port associated with membrane potential changes,
water transport is normally evaluated from the osmoti-
cally induced changes of the cell volume, and such
measurements are often complicated . Further-
more, a background component of water transport is
rather high owing to universal abundance of water and
its rapid diffusion through the lipid bilayer.
The oocytes of a frog
played an important
role in studying water carriers . The membranes of
these cells feature a very low intrinsic permeability to
water. When mRNA is injected into the oocyte, it is nor-
mally expressed as a functional protein. After the injec-
gene mRNA, the water permeability of the
oocyte membrane increased manyfold; the cell swelled
rapidly in a hypoosmotic buffer and lysed. By compar-
ing the kinetics of cell volume changes in untreated and
-expressing oocytes, it was possible to assess the
activity of the protein examined.
Another important approach to studying aquaporins
is the evaluation of their activity in proteoliposomes
The investigation of aquaporins led to several
important conclusions. (1) Many aquaporins signiﬁ-
cantly reduce the activation energy for the transmem-
brane water transfer. The rate of water movement
across the channel approaches to the diffusion rate in
bulk water [17, 18]. (2) Aquaporins provide for bidirec-
tional passage of water, and the transport direction is
determined by physical conditions of the medium [19,
20]. (3) Water transport is passive: it does not require
Aquaporins: Structure, Systematics, and Regulatory Features
A. Yu. Shapiguzov
Timiryazev Institute of Plant Physiology, Russian Academy of Sciences, Botanicheskaya ul. 35, Moscow, 127276 Russia;
fax: 7 (095) 977-9372; e-mail: email@example.com
Received May 13, 2003
—The review describes current views on the molecular structure, systematics, and functional regula-
tion of aquaporins. These recently discovered channel proteins play a principal role in water transport across
cell membranes in the majority of living organisms.
Key words: aquaporins - cell membranes - water relations