Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

CELL JUNCTIONS AND THE EXTRACELLULAR MatrIX
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plasma membranes
CELL 1 CELL 2
cytoskeletal
filaments
Figure 19–3 Transmembrane adhesion
proteins link the cytoskeleton to
extracellular structures. The external
linkage may be either to other cells (cell–cell
junctions, mediated typically by cadherins)
or to extracellular matrix (cell–matrix
junctions, mediated typically by integrins).
The internal linkage to the cytoskeleton is
generally indirect, via intracellular adaptor
proteins, to be discussed later.
extracellular matrix
intracellular
adaptor proteins
transmembrane
adhesion proteins
Two other types of cell –cell junction are shown in Figure 19–2. Tight junctions
hold the cells closely together near the apex, sealing the gap between the cells and
thereby preventing molecules from leaking across the epithelium. Near the basal
end of the cells are channel-forming MBoC6 m19.04/19.03 junctions, called gap junctions, that create
passageways linking the cytoplasms of adjacent cells.
Each of the four major anchoring junction types depends on transmembrane
adhesion proteins that span the plasma membrane, with one end linking to the
cytoskeleton inside the cell and the other end linking to other structures outside it
(Figure 19–3). These cytoskeleton-linked transmembrane proteins fall neatly into
two superfamilies, corresponding to the two basic kinds of external attachment.
Proteins of the cadherin superfamily chiefly mediate attachment of cell to cell
(Movie 19.1). Proteins of the integrin superfamily chiefly mediate attachment of
cells to matrix. There is specialization within each family: some cadherins link to
actin and form adherens junctions, while others link to intermediate filaments
and form desmosomes; likewise, some integrins link to actin and form actinlinked
cell–matrix junctions, while others link to intermediate filaments and form
hemidesmosomes (Table 19–1).
Table 19–1 Anchoring Junctions
Junction
Transmembrane
adhesion protein
Extracellular ligand
Intracellular
cytoskeletal
attachment
Intracellular adaptor
proteins
Cell–Cell
Adherens junction Classical cadherins Classical cadherin on
neighboring cell
Actin filaments
α-Catenin, β-catenin,
plakoglobin (γ-catenin),
p120-catenin, vinculin
Desmosome
Nonclassical
cadherins
(desmoglein,
desmocollin)
Desmoglein and
desmocollin on
neighboring cell
Intermediate filaments
Plakoglobin (γ-catenin),
plakophilin, desmoplakin
Cell–Matrix
Actin-linked cell–
matrix junction
Integrin
Extracellular matrix
proteins
Actin filaments
Talin, kindlin, vinculin,
paxillin, focal adhesion
kinase (FAK), numerous
others
Hemidesmosome
α 6 β 4 Integrin, type
XVII collagen
Extracellular matrix
proteins
Intermediate filaments
Plectin, BP230