Biochemical Society Symposia最新文献

A dramatic rise in intracellular calcium plays a vital role at the moment of fertilization, eliciting the resumption of meiosis and the initiation of embryo development. In mammals, the rise takes the form of oscillations in calcium concentration within the egg, driven by an elevation in inositol trisphosphate. The causative agent of these oscillations is proposed to be a recently described phosphoinositide-specific phospholipase C, PLCzeta, a soluble sperm protein that is delivered into the egg following membrane fusion. In the present review, we examine some of the distinctive structural and functional characteristics of this crucial enzyme that sets it apart from the other known forms of mammalian PLC.

The original hypothesis put forth by Bob Michell in his seminal 1975 review held that inositol lipid breakdown was involved in the activation of plasma membrane calcium channels or 'gates'. Subsequently, it was demonstrated that while the interposition of inositol lipid breakdown upstream of calcium signalling was correct, it was predominantly the release of Ca2+ that was activated, through the formation of Ins(1,4,5)P3. Ca2+ entry across the plasma membrane involved a secondary mechanism signalled in an unknown manner by depletion of intracellular Ca2+ stores. In recent years, however, additional non-store-operated mechanisms for Ca2+ entry have emerged. In many instances, these pathways involve homologues of the Drosophila trp (transient receptor potential) gene. In mammalian systems there are seven members of the TRP superfamily, designated TRPC1-TRPC7, which appear to be reasonably close structural and functional homologues of Drosophila TRP. Although these channels can sometimes function as store-operated channels, in the majority of instances they function as channels more directly linked to phospholipase C activity. Three members of this family, TRPC3, 6 and 7, are activated by the phosphoinositide breakdown product, diacylglycerol. Two others, TRPC4 and 5, are also activated as a consequence of phospholipase C activity, although the precise substrate or product molecules involved are still unclear. Thus the TRPCs represent a family of ion channels that are directly activated by inositol lipid breakdown, confirming Bob Michell's original prediction 30 years ago.

PH (pleckstrin homology) domains represent the 11th most common domain in the human proteome. They are best known for their ability to bind phosphoinositides with high affinity and specificity, although it is now clear that less than 10% of all PH domains share this property. Cases in which PH domains bind specific phosphoinositides with high affinity are restricted to those phosphoinositides that have a pair of adjacent phosphates in their inositol headgroup. Those that do not [PtdIns3P, PtdIns5P and PtdIns(3,5)P2] are instead recognized by distinct classes of domains including FYVE domains, PX (phox homology) domains, PHD (plant homeodomain) fingers and the recently identified PROPPINs (b-propellers that bind polyphosphoinositides). Of the 90% of PH domains that do not bind strongly and specifically to phosphoinositides, few are well understood. One group of PH domains appears to bind both phosphoinositides (with little specificity) and Arf (ADP-ribosylation factor) family small G-proteins, and are targeted to the Golgi apparatus where both phosphoinositides and the relevant Arfs are both present. Here, the PH domains may function as coincidence detectors. A central challenge in understanding the majority of PH domains is to establish whether the very low affinity phosphoinositide binding reported in many cases has any functional relevance. For PH domains from dynamin and from Dbl family proteins, this weak binding does appear to be functionally important, although its precise mechanistic role is unclear. In many other cases, it is quite likely that alternative binding partners are more relevant, and that the observed PH domain homology represents conservation of structural fold rather than function.

Three large protein complexes known as ESCRT I, ESCRT II and ESCRT III drive the progression of ubiquitinated membrane cargo from early endosomes to lysosomes. Several steps in this process critically depend on PtdIns3P, the product of the class III phosphoinositide 3-kinase. Our work has provided insights into the architecture, membrane recruitment and functional interactions of the ESCRT machinery. The fan-shaped ESCRT I core and the trilobal ESCRT II core are essential to forming stable, rigid scaffolds that support additional, flexibly-linked domains, which serve as gripping tools for recognizing elements of the MVB (multivesicular body) pathway: cargo protein, membranes and other MVB proteins. With these additional (non-core) domains, ESCRT I grasps monoubiquitinated membrane proteins and the Vps36 subunit of the downstream ESCRT II complex. The GLUE (GRAM-like, ubiquitin-binding on Eap45) domain extending beyond the core of the ESCRT II complex recognizes PtdIns3P-containing membranes, monoubiquitinated cargo and ESCRT I. The structure of this GLUE domain demonstrates that it has a split PH (pleckstrin homology) domain fold, with a non-typical phosphoinositide-binding pocket. Mutations in the lipid-binding pocket of the ESCRT II GLUE domain cause a strong defect in vacuolar protein sorting in yeast.

Phosphoinositides are minor constituents of cell membranes playing a critical role in the regulation of many cellular functions. Recent discoveries indicate that mutations in several phosphoinositide kinases and phosphatases generate imbalances in the levels of phosphoinositides, thereby leading to the development of human diseases. Although the roles of phosphoinositide 3-kinase products and PtdIns(4,5)P2 were largely studied these last years, the potential role of phosphatidylinositol monophosphates as direct signalling molecules is just emerging. PtdIns5P, the least characterized phosphoinositide, appears to be a new player in cell regulation. This review will summarize the current knowledge on the mechanisms of synthesis and degradation of PtdIns5P as well as its potential roles.

PtdIns(3,5)P2 was discovered about a decade ago and much of the machinery that makes, degrades and senses it has been uncovered. Despite this, we still lack a complete understanding of how the pieces fit together but some patterns are beginning to emerge. Molecular functions for PtdIns(3,5)P2 are also elusive, but the identification of effectors offers a way into some of these processes. An examination of the defects associated with loss of synthesis of PtdIns(3,5)P2 in lower and higher eukaryotes begins to suggest a unifying theme; this lipid regulates membrane retrieval via retrograde trafficking from distal compartments to organelles that are more proximal in the endocytic/lysosomal system. Another unifying theme is stress signalling to organelles, possibly both to change their morphology in response to external insults and to maintain the lumenal pH or membrane potential of organelles. The next few years seem likely to uncover details of the molecular mechanisms underlying the biology of this fascinating lipid. This review also highlights some areas where further research is needed.

Phosphoinositide signals regulate cell proliferation, differentiation, cytoskeletal rearrangement and intracellular trafficking. Hydrolysis of PtdIns(4,5)P2 and PtdIns(3,4,5)P3, by inositol polyphosphate 5-phosphatases regulates synaptic vesicle recycling (synaptojanin-1), hematopoietic cell function [SHIP1(SH2-containing inositol polyphosphate 5-phosphatase-1)], renal cell function [OCRL (oculocerebrorenal syndrome of Lowe)] and insulin signalling (SHIP2). We present here a detailed review of the characteristics of the ten mammalian 5-phosphatases. Knockout mouse phenotypes and underexpression studies are associated with significant phenotypic changes, indicating non-redundant roles, despite, in many cases, overlapping substrate specificity and tissue expression. The extraordinary complexity in the control of phosphoinositide signalling continues to be revealed.

The IP3R [IP3 (inositol 1,4,5-trisphosphate) receptor] is responsible for Ca2+ release from the ER (endoplasmic reticulum). We have been working extensively on the P400 protein, which is deficient in Purkinje-neuron-degenerating mutant mice. We have discovered that P400 is an IP3R and we have determined the primary sequence. Purified IP3R, when incorporated into a lipid bilayer, works as a Ca2+ release channel and overexpression of IP3R shows enhanced IP3 binding and channel activity. Addition of an antibody blocks Ca2+ oscillations indicating that IP3R1 works as a Ca2+ oscillator. Studies on the role of IP3R during development show that IP3R is involved in fertilization and is essential for determination of dorso-ventral axis formation. We found that IP3R is involved in neuronal plasticity. A double homozygous mutant of IP3R2 (IP3R type 2) and IP3R3 (IP3R type 3) shows a deficit of saliva secretion and gastric juice secretion suggesting that IP3Rs are essential for exocrine secretion. IP3R has various unique properties: cryo-EM (electron microscopy) studies show that IP3R contains multiple cavities; IP3R allosterically and dynamically changes its form reversibly (square form-windmill form); IP3R is functional even though it is fragmented by proteases into several pieces; the ER forms a meshwork but also forms vesicular ER and moves along microtubules using a kinesin motor; X ray analysis of the crystal structure of the IP3 binding core consists of an N-terminal beta-trefoil domain and a C-terminal alpha-helical domain. We have discovered ERp44 as a redox sensor in the ER which binds to the luminal part of IP3R1 and regulates its activity. We have also found the role of IP3 is not only to release Ca2+ but also to release IRBIT which binds to the IP3 binding core of IP3R.

InsP3 has two important functions in generating Ca2+ oscillations. It releases Ca2+ from the internal store and it can contribute to Ca2+ entry. A hypothesis has been developed to describe a mechanism for Ca2+ oscillations with particular emphasis on the way agonist concentration regulates oscillator frequency. The main idea is that the InsP3 receptors are sensitized to release Ca2+ periodically by cyclical fluctuations of Ca2+ within the lumen of the endoplasmic reticulum. Each time a pulse of Ca2+ is released, the luminal level of Ca2+ declines and has to be replenished before the InsP3 receptors are resensitized to deliver the next pulse of Ca2+. It is this loading of the internal store that explains why frequency is sensitive to external Ca2+ and may also account for how variations in agonist concentration are translated into changes in oscillation frequency. Variations in agonist-induced entry of external Ca2+, which can occur through different mechanisms, determine the variable rates of store loading responsible for adjusting the sensitivity of the InsP3 receptors to produce the periodic pulses of Ca2+. The Ca2+ oscillator is an effective analogue-to-digital converter in that variations in the concentration of the external stimulus are translated into a change in oscillator frequency.

The mature sphingolipids of yeast consist of IPCs (inositolphosphorylceramides) and glycosylated derivatives thereof. Beyond being an abundant membrane constituent in the organelles of the secretory pathway, IPCs are also used to constitute the lipid moiety of the majority of GPI (glycosylphosphatidylinositol) proteins, while a minority of GPI proteins contain PI (phosphatidylinositol). Thus all GPI anchor lipids (as well as free IPCs) typically contain C26 fatty acids. However, the primary GPI lipid that isadded to newly synthesized proteins in the endoplasmic reticulum consists of a PI with conventional C16 and C18 fatty acids. A new class of enzymes is required to replace the fatty acid in sn-2 by a C26 fatty acid. Cells lacking this activity make normal amounts of GPI proteins but accumulate GPI anchors containing lyso-PI. As a consequence, the endoplasmic reticulum to Golgi transport of the GPI protein Gas1p is slow, and mature Gas1p is lost from the plasma membrane into the medium. The GPI anchor containing C26 in sn-2 can further be remodelled by the exchange of diacylglycerol for ceramide. This process is also dependent on the presence of specific phosphorylethanolamine side-chains on the GPI anchor.