Ganong's Review of Medical Physiology, 23rd Edition

COATS & VESICLE TRANSPORT
It now appears that all vesicles involved in transport have protein
coats. In humans, 53 coat complex subunits have been
identified. Vesicles that transport proteins from the trans Golgi
to lysosomes have assembly protein 1 (AP-1) clathrin
coats, and endocytotic vesicles that transport to endosomes
have AP-2 clathrin coats. Vesicles that transport between the
endoplasmic reticulum and the Golgi have coat proteins I and
II (COPI and COPII). Certain amino acid sequences or attached
groups on the transported proteins target the proteins
for particular locations. For example, the amino acid sequence
Asn–Pro–any amino acid–Tyr targets transport from the cell
surface to the endosomes, and mannose-6-phosphate groups
target transfer from the Golgi to mannose-6-phosphate receptors
(MPR) on the lysosomes.
Various small G proteins of the Rab family are especially
important in vesicular traffic. They appear to guide and facilitate
orderly attachments of these vesicles. To illustrate the
complexity of directing vesicular traffic, humans have 60 Rab
proteins and 35 SNARE proteins.
MEMBRANE PERMEABILITY &
MEMBRANE TRANSPORT PROTEINS
An important technique that has permitted major advances in
our knowledge about transport proteins is patch clamping. A
micropipette is placed on the membrane of a cell and forms a
tight seal to the membrane. The patch of membrane under the
pipette tip usually contains only a few transport proteins, allowing
for their detailed biophysical study (Figure 2–14). The
cell can be left intact (cell-attached patch clamp). Alternatively,
the patch can be pulled loose from the cell, forming an
inside-out patch. A third alternative is to suck out the patch
with the micropipette still attached to the rest of the cell membrane,
providing direct access to the interior of the cell (whole
cell recording).
Small, nonpolar molecules (including O 2 and N 2 ) and small
uncharged polar molecules such as CO 2 diffuse across the
lipid membranes of cells. However, the membranes have very
limited permeability to other substances. Instead, they cross
the membranes by endocytosis and exocytosis and by passage
through highly specific transport proteins, transmembrane
proteins that form channels for ions or transport substances
such as glucose, urea, and amino acids. The limited permeability
applies even to water, with simple diffusion being supplemented
throughout the body with various water channels
(aquaporins). For reference, the sizes of ions and other biologically
important substances are summarized in Table 2–2.
Some transport proteins are simple aqueous ion channels,
though many of these have special features that make them
selective for a given substance such as Ca 2+ or, in the case of
aquaporins, for water. These membrane-spanning proteins
(or collections of proteins) have tightly regulated pores that
can be gated opened or closed in response to local changes
CHAPTER 2 Overview of Cellular Physiology in Medical Physiology 45
Electrode
Pipette
Cell
membrane
Closed
Open
Inside-out patch
FIGURE 2–14 Patch clamp to investigate transport. In a patch
clamp experiment, a small pipette is carefully maneuvered to seal off a
portion of a cell membrane. The pipette has an electrode bathed in an
appropriate solution that allows for recording of electrical changes
through any pore in the membrane (shown below). The illustrated setup
is termed an “inside-out patch” because of the orientation of the membrane
with reference to the electrode. Other configurations include cell
attached, whole cell, and outside-out patches. (Modified from Ackerman
MJ, Clapham DE: Ion channels: Basic science and clinical disease. N Engl J Med
1997;336:1575.)
(Figure 2–15). Some are gated by alterations in membrane
potential (voltage-gated), whereas others are opened or
closed in response to a ligand (ligand-gated). The ligand is
TABLE 2–2 Size of hydrated ions and other substances
of biologic interest.
Substance Atomic or Molecular Weight Radius (nm)
Cl – 35 0.12
K + 39 0.12
H 2 O 18 0.12
Ca 2+ 40 0.15
Na + 23 0.18
Urea 60 0.23
Li + 7 0.24
Glucose 180 0.38
Sucrose 342 0.48
Inulin 5000 0.75
Albumin 69,000 7.50
Data from Moore EW: Physiology of Intestinal Water and Electrolyte Absorption.
American Gastroenterological Association, 1976.
ms
pA