Contractile Vacuole Complex—Its Expanding Protein Inventory - International Review of Cell and Molecular Biology - page 390

Dictyostelium —remarkably independent of the adaptor protein AP-2 ( Macro
et al., 2012 ). (Note that in higher eukaryotes, AP-2 mediates the assembly of
clathrin-coated vesicles at the cell membrane in cooperation with several
other proteins ( Reider and Wendland, 2011 ).) This is in contrast to dis-
turbed osmoregulation after knockout of AP-2 SUs ( Wen et al., 2009 ).
In Dictyostelium , knockouts of clathrin heavy ( O'Halloran and Anderson,
1992 ) or light chains ( Wang et al., 2003 ), of the adaptor protein AP-1
( Lefkir et al., 2003 ) or AP-180 ( Stavrou and O'Halloran, 2006 ) each result
in disturbed osmoregulation. Therefore, in Dictyostelium , clathrin-based
vesicular transport, be it from the Golgi apparatus or from the cell mem-
brane, appears well documented.
The distribution of copine A (a C2-domain containing Ca 2 þ -binding
protein) in Dictyostelium over cell membrane, CV membrane as well as
endosomal and phagosomal membranes ( Damer et al., 2005 ) has also been
considered as evidence of such input. Similarly seductive, but unreliable are
some observations concerning an input by phagocytosis. As cited by Harris
et al. (2002) “the CV network and phagocytosis have also been linked in
Tetrahymena pyriformis . A 71-kDa protein, associated with the actin-binding
proteins, localized to both the CV and oral apparatus ... suggesting that a
connection may exist between the membranes involved for internalization
and osmoregularity ... ( Watanabe et al., 1998 )”. However, the fact is that
particles ingested by phagocytosis are never seen, for example, in the CV
of Paramecium .
This can be summarized as follows. Whereas all this clearly documents
the relevance of an endocytotic input into the CVC of Dictyostelium , no such
data are available for ciliates. The same holds for the relevance of clathrin-
coated pits and vesicles. Never seen in the CVC, for example, of Paramecium ,
they show up nicely at the EM level in Dictyostelium ( Heuser, 2006 ) where,
by molecular tools, input via clathrin-coated vesicles has been documented.
Any possible input of membrane material into the CV by phagocytosis
remains questionable.

6. THE CV PORE AND EPIGENETIC ASPECTS
OF ORGANELLE POSITIONING
6.1. Components of the CV pore
The pore is the site where the CV fuses with the cell membrane in a kiss-
and-run type exocytosis. In Paramecium , the site containing the pore is a

1- m m wide depression where the CV membrane is intimately attached