BIO130H1 Lecture Notes - Lecture 22: Adherens Junction, Tight Junction, Basal Lamina

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BIO130H1 - Lecture 22 – Junctions
Thursday, March 31 st
, 2016
In polarized epithelial cells:
-Junctions are arranged in a specific order
-Other types of cells can have junctions in different orders, but this is the order in
epithelial cells:
-Tight junction, adherens junction, desmosome, gap junction
oTight junctions
Function as occluding junctions
Keep transmembrane proteins on the right side
They form sealing strands (tight junction belts)
oAdherens junctions
Basal to the tight junctions
Type of anchoring junction
Forms adhesion belt between two cells
oDesmosome
Another type of anchoring junction
Basal to adherens junctions
Anchors at a specific spot, not all along the membrane
oGap junctions
Channel-forming junctions
Basal to desmosome
Not channel to outside, channels between cells
oHemidesmosomes
At the very bottom
Kind of like half a desmosome anchored to the extracellular matrix
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Anchored to a special ECM, the basal lamina
-Polarized means ends are different
-Apical side has microvilli
Tight Junctions
-Create a tight seal between cells
-Prevent mixing of the extracellular environments
-Act as fences in the membrane
-Prevent mixing of membrane proteins
oApical membrane proteins
oBasal membrane proteins
Epithelial Sheets are a barrier
-The most apical junction is a tight junction on epithelial cells
-So if you trace a dye, it will stay on apical side since it is unable to pass through
the tight junctions (apical-most junction)
-In plainer terms, since the tight junctions are the outermost “door,” it keeps out
the external substances
Claudin and Occludin proteins form the seal
-Transmembrane proteins
-Extracellular domains
oInteract with the extracellular domains of the proteins in the neighbouring
cell
Occludin/occludin
Claudin/claudin
oClaudin:
Required for tight junction seals
Functional proteins are required in both cells
-In this case, both N- and C-terminus on cytosolic side
Tight Junctions and Adherens Junctions
-Tight junctions are dependent on adherens junctions
-If you block the formation of adherens junctions, tight junctions don’t form
properly
-Junctional complex
oComprised of tight junctions, adherens junction, and desmosome
-Tight junctions can provide a bit of mechanical strength
Anchoring Junctions
-Includes adherens junctions, desmosomes, and hemidesmosomes
-Cell-cell anchoring junctions
-Cell-matrix anchoring junctions
-Link the cytoskeletons of neighbouring cells
-Different proteins are involved in different types of interactions
Adhesion and Anchor Proteins
-Link cytoskeletal filaments of neighbouring cells
-Adhesion proteins
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