Regulation of the intracellular localization of phosphoinositide-specific phospholipase Cdelta(1)

Phospholipase C Isoforms Are Localized at the Cleavage Furrow during Cytokinesis

It has recently been demonstrated that phosphatidylinositol 4,5-bisphosphate (PIP2) is localized at the cleavage furrow in dividing cells and its hydrolysis is required for complete cytokinesis, suggesting a pivotal role of PIP2 in cytokinesis. Here, we report that at least three mammalian isoforms of phosphoinositide-specific phospholipase C (PLC), PLCdelta1, PLCdelta3 and PLCbeta1, are localized to the cleavage furrow during cytokinesis. Targeting of the delta1 isoform to the furrow depends on the specific interaction between the PH domain and PIP2 in the plasma membrane. The necessity of active PLC in animal cell cytokinesis was confirmed using the specific inhibitors for PIP2 hydrolysis. These results support the model that activation of selected PLC isoforms at the cleavage furrow controls progression of cytokinesis through regulation of PIP2 levels: induction of the cleavage furrow by a contractile ring consisting of actomyosin is regulated by PIP2-dependent actin-binding proteins and formation of specific lipid domains required for membrane separation is affected by alterations in the lipid composition of the furrow.

Phosphatidylserine induces functional and structural alterations of the membrane-associated pleckstrin homology domain of phospholipase C-δ1

The membrane binding affinity of the pleckstrin homology (PH) domain of phospholipase C (PLC)-delta1 was investigated using a vesicle coprecipitation assay and the structure of the membrane-associated PH domain was probed using solid-state (13)C NMR spectroscopy. Twenty per cent phosphatidylserine (PS) in the membrane caused a moderate but significant reduction of the membrane binding affinity of the PH domain despite the predicted electrostatic attraction between the PH domain and the head groups of PS. Solid-state NMR spectra of the PH domain bound to the phosphatidylcholine (PC)/PS/phosphatidylinositol 4,5-bisphosphate (PIP(2)) (75 : 20 : 5) vesicle indicated loss of the interaction between the amphipathic alpha2-helix of the PH domain and the interface region of the membrane which was previously reported for the PH domain bound to PC/PIP(2) (95 : 5) vesicles. Characteristic local conformations in the vicinity of Ala88 and Ala112 induced by the hydrophobic interaction between the alpha2-helix and the membrane interface were lost in the structure of the PH domain at the surface of the PC/PS/PIP(2) vesicle, and consequently the structure becomes identical to the solution structure of the PH domain bound to d-myo-inositol 1,4,5-trisphosphate. These local structural changes reduce the membrane binding affinity of the PH domain. The effects of PS on the PH domain were reversed by NaCl and MgCl(2), suggesting that the effects are caused by electrostatic interaction between the protein and PS. These results generally suggest that the structure and function relationships among PLCs and other peripheral membrane proteins that have similar PH domains would be affected by the local lipid composition of membranes.

Coordinated intracellular translocation of phosphoinositide-specific phospholipase C-δ with the cell cycle

The delta family phosphoinositide (PI)-specific phospholipase C (PLC) are most fundamental forms of eukaryotic PI-PLCs. Despite the presence of lipid targeting domains such as the PH domain and C2 domain, the isoforms are also found in the cytoplasm and nucleus as well as at the plasma membrane. The isoforms have sequences or regions that can serve as a nuclear localization signal (NLS) and a nuclear export signal (NES). Their intracellular localization differs from one isoform to another, presumably due to the difference in the transport equilibrium balanced by the strength of the two signals of each isoform. Even for a particular isoform, its intracellular localization seems to vary during the cell cycle. As an example, PLCdelta(1), which is generally found at the plasma membrane and in the cytoplasm of quiescent cells, localizes to discrete nuclear structures in the G(1)/S boundary of the cell cycle. This may be at least partly due to an increase in intracellular Ca(2+), since Ca(2+) facilitates the formation of a nuclear transport complex comprised of PLCdelta(1) and importin beta1, a carrier molecule for the nuclear import. PLCdelta(1) as well as PLCdelta(4) may play a pivotal role in controlling the initiation of DNA synthesis in S phase. Spatio-temporal changes in the levels of PtdIns(4,5)P(2) seem to be another major determinant for the localization and regulation of the delta isoforms. High nuclear PtdIns(4,5)P(2) levels are associated with the G(1)/S phases. After entering M phase, PtdIns(4,5)P(2) synthesis at sites of cell division occurs and PLCs seem to localize to the cleavage furrow during cytokinesis. Coordinated translocation of PLCs with the cell cycle or with stress responses may result in changes in intra-nuclear environments and local membrane architectures that modulate proliferation and differentiation. In this review, recent findings regarding the molecular machineries and mechanisms of the nucleocytoplasmic shuttling as well as roles in the cell cycle progression of the delta isoforms of PLC will be discussed.

Nucleocytoplasmic shuttling of phospholipase C-δ1: A link to Ca2+

Phosphoinositides (PIs) and proteins involved in the PI signaling pathway are distributed in the nucleus as well as at the plasma membrane and in the cytoplasm, although their nuclear localization mechanisms have not been clarified in detail. Generally, proteins that shuttle between the cytoplasm and nucleus contain nuclear localization signal (NLS) and nuclear export signal (NES) sequences for nuclear import and export, respectively. They bind to specific carrier proteins of the importin/exportin family and are transported to and from the nucleus. Thus there is a steady state shuttling of the cargo molecules to and from the nucleus, and the shift in equilibrium determines their nuclear or cytoplasmic localization. Our previous studies have shown that phospholipase C (PLC)-delta1, regarded as having cytoplasmic- or plasma membrane-bound localization, accumulates in the nucleus when its NES sequence is disrupted. In addition, a cluster of positively charged residues on the surface of the catalytic barrel is important for nuclear import. In quiescent cells, the shuttling equilibrium seems to be shifted to the nuclear export of PLCdelta1. In this review, recent findings regarding the molecular machineries and mechanisms of the nucleocytoplasmic shuttling of PLCdelta1 will be discussed. It is important to know when and how they are regulated. A shift in the equilibrium in a certain stage of the cell cycle or by external stimuli is possible and resulting changes in the intra-nuclear environments (or architectures) may alter proliferation and differentiation patterns. Evidences support the idea that an increase in the levels of intracellular Ca2+ shifts the equilibrium to the nuclear import of PLCdelta1. A myriad of external stimuli have also been reported to change the nuclear PI metabolism following accelerated accumulation in the nucleus of other phospholipases such as phospholipase A2 and phospholipase D in addition to PLC isoforms such as PLCbeta1 and PLCgamma1. The consequence of the nuclear accumulation of PLC is also discussed.