Annexins and Membrane Fusion - Fusion of Biological Membranes and Related Problems - page 96

associated with spectrin and do not require annexin VI for budding. So it
appears that there are two types of coated vesicle budding, one that requires
the activation of a cysteine protease to release clathrin from membrane-
bound spectrin during budding, and another that is independent of this
process (Kamal et al., 1998). In this study annexin VI seems to play a more
important role in guiding the endosomes to their correct location rather
than for pit budding. It is also possible that annexin VI becomes coupled
with a protease that is needed to release the spectrin and thus the annexin
VI targets it to the area of membrane where it is needed for coated pit
budding. Spectrin is not found on all membranes where coated pits are
formed, instead other parts of the cytoskeleton play a part, an example
being on the apical surface of polarized epithelial cells where there is vir-
tually no spectrin. When actin microfilaments become depolymerized pit
budding is inhibited on the apical membrane, but the spectrin covered baso-
lateral membrane is still able to form endosomes (Gottlieb et al., 1993).
These results tend to eliminate annexin VI from having a role in the
budding of clathrin coated pits, but instead point to a role for annexin VI
in an endocytic pathway that is presumably dispensable in A431 cells.
3.2. Annexin II
Annexin II exists in many subcellular locations depending on cell type
and expression of it's intracellular ligand p11. There is almost invariably a
pool of annexin II at the plasma membrane, it may be expressed on the cell
surface, it is also enriched on early endosomes, it has been reported to be
present in coated pitsb/vesicles by immunogold labelling of cryosections, and
there are conflicting data regarding the association of annexin II with cave-
olae (Gruenberg and Emans, 1993). The submembraneous pool of annexin
II that exists in a stable association with the actin cytoskeleton acquires
this localization by forming a heterotetramer with p11 (Gerke and Weber,
1985). As discussed earlier in the context of exocytosis, binding of p11 to
annexin II increases the affinity of annexin II for Ca 2+ and phospholipids
(Powell and Glenney, 1987). Monomeric annexin I1 is invariably cytosolic
but binding to p11 in the tetrameric configuration seems to target the
complex to the cytoskeleton (Thiel et al., 1992). In most cells it appears that
annexin II is present in molar excess over p11. Because the affinity of
annexin II for p11 is high, all the cellular p11 becomes recruited into the
complex and the cytosolic pool of monomeric annexin II simply corre-
sponds to the intracellular molar excess. Annexin II is an in vivo substrate
for PKC (which phosphorylates annexin II on serine-25) (Gould et al., 1986)
and the transforming gene product of the Rous sarcoma virus (which phos-
phorylates annexin II on tyrosine-23) (Glenney, 1986). Phosphorylation of