Biology Reference
In-Depth Information
Introduction
A unique feature of intestinal epithelial cells is the presence of a brush border
composed of numerous membrane extensions called microvilli, fingerlike
extensions which play a key role in the absorptive function of these specialized
cells. They are abundant on those epithelial cells that require a very large
surface area to function e ciently. Each microvillus is about 0.08 mm wide and
1 mm long, making the cell's absorptive area 20 times greater than it would be
without them. The plasma membrane that covers these microvilli is highly
specialized, bearing a thick extracellular coat of polysaccharide and digestive
enzymes. The core of each intestinal microvillus is a rigid bundle of 20 to 30
parallel actin filaments that extend from the tip of the microvillus down into
the cell cortex. The actin filaments in the bundle are all orientated with their
plus ends pointing away from the cell body. They are held together at regular
intervals by actin-bundling proteins. Fimbrin, a ubiquitous actin binding
protein, is involved in actin microfilament organization in microspikes and
filipodia and it also contribute to the bundling of actin filaments into
microvilli. By contrast, villin is only found in microvilli of the digestive and
urinary tracts where it is known to bundle actin filaments, thus playing a key
role in the maintenance of brush border architecture.
Villin, a structural actin-binding protein
Structure and function
Villin, first isolated from chicken intestinal epithelial cells and later from
mammalian species, is an acidic polypeptide with a molecular mass of
92.5 kDa, occurring in monomeric form. Villin is only detected in a few epi-
thelial cells from the gastrointestinal, the urogenital and the respiratory tracts.
The gelsolin family to which villin belongs contains several members:
gelsolin, adseverin, CapG, advillin and supervillin (Figure 15.1). These
proteins all contain three to six homologous evolutionarily conserved
domains. The typical villins have six of these and in addition they have a
so-called 'head piece' at the C-terminus that is the largest single difference
between gelsolin and villins (for review see Friederich et al., 1990). Villin
(Northrop et al., 1986), gelsolin (Kwiatkowski and Yin, 1987), adseverin
(Lueck et al., 1998) and CapG (Mishra et al., 1994) have the common
property of binding to barbed ends of actin filaments with high a nity. Villin,
gelsolin and adseverin can sever actin filaments whereas CapG lacks this
activity (Southwick and DiNubile, 1986). Among all actin-binding proteins,
villin is unique in presenting capping, bundling and severing properties in a
single protein.
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