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VOL. 73 (4), 901-925, 2007 ROLES OF PROTEIN PHOSPHATASE TYPE 1...

Scd5 targets PP1 to actin patches and vesicles, where the Glc7/Scd5
holoenzyme counteracts Prk1 phosphorylation being potential
substrates of Glc7/Scd5 the phosphoproteins Sla2, Sla1 and Pan1
(73, 74). As Prk1 phosphorylation acts at multiple levels to regulate
actin dynamics and the association of endocytic proteins a dynamic
interplay between Scd5/Glc7 and Prk1 is likely to be critical to
control endocytosis.

Although endocytosis is less extensively characterised in fission
yeast, where it has been studied, it mirrors that of budding yeast.
The fission yeast Sla2/End4 gene product localises in the identical
manner to its budding yeast orthologue and is similarly required for
endocytosis (75). By analogy with the three phases in budding yeast,
we found that Dis2 associates with vesicles during the non-motile
phase and leaves just prior to the highly motile one. It was recruited
to the patches after Sla2/End4. As has been previously reported for
Sla2/End4, de-polymerisation of actin with LatA did not block
recruitment of Dis2 to the cell cortex but did block Dis2.NEGFP foci
at the cell cortex before internalisation (76-78).

POSSIBLE FUNCTIONS OF DIS 2 IN POLARIZED
CELL GROWTH

The recruitment of a second population of Dis2.NEGFP foci to
cell tips by Wsh3/Tea4 in a manner that is dependent upon the Kelch
domain protein Tea1 is highly reminiscent of budding yeast, where
Bud14 protein relies upon the Kelch domain proteins Kel1 and Kel2
to recruit Glc7 to the bud tip to control dynein activity during spindle
orientation. The S. pombe Bud14 homologue, Wsh3/Tea4 (26, 27)
both co-localised with Dis2 and physically associated with it. The
binding of PP1 to Wsh3/Tea4 is critically important for the role
played by Wsh3/Tea4 in maintaining polarised tip growth as cells
transit the cell cycle or recover from osmotic stress. This function is
likely to involve modulation of the F-actin cytoskeleton as the ability
of Wsh3/Tea4 to promote F-actin polymerisation is compromised
when it cannot bind PP1 (Figure 6).

Thus, like its A. nidulans homologue BimG, Dis2 function is
required for tip growth. The requirement for PP1 function during

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