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ISABEL ÁLVAREZ-TABARÉS Y COLS. ANAL. REAL ACAD. NAC. FARM.

The site of endocytosis is determined in a non-motile phase by
the co-operative action of molecules such as clathrin, Sla1, Sla2 and
Pan1. The association of these proteins with the emerging vesicle
persists through the next, slow motile, phase during which the
recruitment of myosin 5 and polymerisation of actin accompanies
the formation of the clathrin pit. The vesicle then enters a rapid
motile phase following the cessation of further actin polymerisation,
the departure of myosin 5 and the scission of the pit to form the
vesicle. Actin polymerisation is accompanied by the recruitment of
homologues of the higher eukaryotic proteins N-WASP and the Arp2/
3 complex which harness actin polymerisation to promote scission
and the eventual long range, fast motility of the free vesicle in the
final phase (60, 61). The early components of the patches (Sla1,
Sla2 and Pan1) are likely to be disassembled as a response
to phosphorylation by the Ser/Thr kinases Ark1 and Prk1 (note that
S. cerevisiae Ark1 is not related to the fission yeast aurora-related
kinase Ark1). Ark1 and Prk1 localise to actin patches in an Abp1-
dependent manner (62) and are thus likely to be components of late
patches. Recent data support a model in which, following the
association of actin with the endocytic machinery, Ark1 and Prk1
can phosphorylate Sla1 and Pan1 and this Prk1-dependent
phosphorylation disrupts the association of Sla1 with Pan1 (63). If
Pan1 and Sla1 are phosphorylated and then lost from the complex,
dephosphorylation might allow their re-incorporation into cortical
complexes. This remains to be shown, as the role of phosphatases in
endocytosis has not been extensively studied. However, interactions
between Sla1 and the yeast homologue of PP1, Glc7 (64, 65), as well
as between Pan1 and Glc7 have been reported (66). Interestingly, the
glc7 conditional mutant glc7.10 has been shown to be defective in
vacuolar fusion and in secretory and endocytic vesicular transport
(67). Similar phenotypes are observed after the functional disruption
of proteins involved in endocytosis such as Scd5 or Sla2 (67, 68).Scd5
has been identified as a PP1-binding protein in various screens (65,
69, 70). Scd5 binds to Glc7 via a classic PP1 binding motif and the
disruption of the RVXF motif of Scd5 severely compromised
endocytosis, actin structure and confers temperature sensitive
lethality. Sla2 interacts physically and genetically with Scd5 (71, 72),
and Scd5 is required in conjunction with clathrin for the association
of Sla2 with cortical actin (73). A possible scenario could be that

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