The SAPs, a New Family of Proteins, Associate and Function Positively with the SIT4 Phosphatase

Abstract

SIT4 is the catalytic subunit of a type 2A-related protein phosphatase in Saccharomyces cerevisiae that is required for G1 cyclin transcription and for bud formation. SIT4 associates with several high-molecular-mass proteins in a cell cycle-dependent fashion. We purified two SIT4-associated proteins, SAP155 and SAP190, and cloned the corresponding genes. By sequence homology, we isolated two additional SAP genes, SAP185 and SAP4. Through such an association is not yet proven for SAP4, each of SAP155, SAP185, and SAP190 physically associates with SIT4 in separate complexes. The SAPs function positively with SIT4, and by several criteria, the loss of all four SAPs is equivalent to the loss of SIT4. The data suggest that the SAPs are not functional in the absence of SIT4 and likewise that SIT4 is not functional in the absence of the SAPs. The SAPs are hyperphoshorylated in cells lacking SIT4, raising the possibility that the SAPs are substrates of SIT4. By sequence similarity, the SAPs fall into two groups, the SAP4/SAP155 group and the SAP185/SAP190 group. Overexpression of a SAP from one group does not suppress the defects due to the loss of the other group. These findings and others indicate that the SAPs have distinct functions.

Identification of phosphatase 2A‐like Sit4‐mediated signalling and ubiquitin‐dependent protein sorting as modulators of caffeine sensitivity in S. cerevisiae

“…The sap155∆, sap4∆, and sap185∆ cells showed normal rapamycin sensitivity; as previously reported (Xie et al, 2005), however, the sap190∆ strain was more resistant to rapamycin than the wild-type strain ( Figure 4A, right panel). The unique caffeine resistance of the ∆sap155 strain is consistent with previous genetic results indicating that Sap155 has a function that is distinct from the other SAPs (Luke et al, 1996). The combined data for the sap∆ strains also highlights differences in the response of cells to caffeine and rapamycin.…”

“…Sit4 function requires a family of proteins termed SAPs for Sit4-associated proteins (Luke et al, 1996). Genetic and/or physical interactions also link Sds23/24 and Rts3 to Sap proteins -the former with Sap155 and the latter with Sap185.…”

Life in the midst of scarcity: adaptations to nutrient availability in Saccharomyces cerevisiae

“…The PP2A holoenzyme contains one of the two redundant catalytic subunits (PP2Ac), Pph21, Pph22 (Sneddon et al 1990;Ronne et al 1991), a scaffolding subunit, Tpd3 (van Zyl et al 1992), and one of the two regulatory subunits, Cdc55 or Rts1 (Healy et al 1991;Zhao et al 1997). The PP2A-related phosphatase is mainly found as a complex between the catalytic subunit, Sit4 (Arndt et al 1989), and one of the four regulatory subunits, Sap4, Sap155, Sap185 and Sap190 (Luke et al 1996). TORC1 controls the activity of these phosphatases via Tap42.…”

Functional analysis of the PP2A subfamily of protein phosphatases in regulating Drosophila S6 kinase

“…The core dimer binds additional regulatory subunits that target PP2A to specific substrates [25,26]. The mammalian PP4 catalytic subunit also interacts with scaffold and regulatory subunits [27,28], while PP6 regulatory proteins have been identified in yeast [29]. Due to decreased complexity relative to mammalian cells, Drosophila has been a valuable system to study functions of individual catalytic and regulatory subunits within the PP2A subfamily [30][31][32].…”