PtdIns(3,4,5)P3 and
PtdIns(3,4)P2 are major signalling molecules in mammalian cell biology.
PtdIns(3,4)P2 can be produced by PI3Ks [PI (
phosphoinositide) 3-kinases], but also by PI 5-phosphatases including SHIP2 [SH2 (Src homology 2)-domain-containing
inositol phosphatase 2]. Proteomic studies in human cells revealed that SHIP2 can be phosphorylated at more than 20 sites, but their individual function is unknown. In a model of
PTEN (phosphatase and
tensin homologue deleted on chromosome 10)-null
astrocytoma cells, lowering SHIP2 expression leads to increased
PtdIns(3,4,5)P3 levels and Akt phosphorylation. MS analysis identified SHIP2 phosphosites on Ser132, Thr1254 and Ser1258;
phosphotyrosine-containing sites were undetectable. By immunostaining, total SHIP2 concentrated in the perinuclear area and in the nucleus, whereas SHIP2 phosphorylated on Ser132 was in the cytoplasm, the nucleus and nuclear speckles, depending on the cell cycle stage. SHIP2 phosphorylated on Ser132 demonstrated
PtdIns(4,5)P2 phosphatase activity. Endogenous phospho-SHIP2 (Ser132) showed an overlap with
PtdIns(4,5)P2 staining in nuclear speckles. SHIP2 S132A was less sensitive to C-terminal degradation and more resistant to
calpain as compared with wild-type
enzyme. We have identified nuclear
lamin A/C as a novel SHIP2 interactor. We suggest that the function of SHIP2 is different at the plasma membrane where it recognizes
PtdIns(3,4,5)P3, and in the nucleus where it may interact with
PtdIns(4,5)P2, particularly in speckles.