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Biomedical subjects

Martin Lowe

Publications and source records attributed to Martin Lowe.

17 recordsLinked to original sources
PubMed2006

Membrane targeting and activation of the Lowe syndrome protein OCRL1 by rab GTPases.

The X-linked disorder oculocerebrorenal syndrome of Lowe is caused by mutation of the OCRL1 protein, an inositol polyphosphate 5-phosphatase. OCRL1 is localised to the Golgi apparatus and early endosomes, and can translocate to lamellipodia upon growth factor stimulation. We show here that OCRL1 interacts with several members of the rab family of small GTPases. Strongest interaction is seen with Golgi-associated rab1 and rab6 and endosomal rab5. Point mutants defective in rab binding fail to target to the Golgi apparatus and endosomes, strongly suggesting rab interaction is required for targeting of OCRL1 to these compartments. Membrane recruitment via rab binding is required for changes in Golgi and endosomal dynamics induced by overexpression of catalytically inactive OCRL1. In vitro experiments demonstrate that rab5 and rab6 directly stimulate the 5-phosphatase activity of OCRL1. We conclude that rabs play a dual role in regulation of OCRL1, firstly targeting it to the Golgi apparatus and endosomes, and secondly, directly stimulating the 5-phosphatase activity of OCRL1 after membrane recruitment.

Amino Acid Sequence↗
PubMed2005

Protein kinase D: activation for Golgi carrier formation.

Protein kinase D regulates fission at the trans-Golgi network (TGN) of transport carriers that deliver cargo to the plasma membrane. PKD is first recruited to the TGN through interaction with diacylglycerol and is subsequently activated by phosphorylation to promote carrier fission. In a recent study, the relevant upstream kinase at the TGN was identified as the novel protein kinase C isoform PKCeta, which in turn is activated in response to heterotrimeric G-protein activation. These findings indicate the existence of a kinase signaling cascade at the TGN that regulates carrier fission and suggest a mechanism by which cargo might direct the formation of its transport carriers.

Animals↗
PubMed2005

Lowe syndrome protein OCRL1 interacts with clathrin and regulates protein trafficking between endosomes and the trans-Golgi network.

Oculocerebrorenal syndrome of Lowe is caused by mutation of OCRL1, a phosphatidylinositol 4,5-bisphosphate 5-phosphatase localized at the Golgi apparatus. The cellular role of OCRL1 is unknown, and consequently the mechanism by which loss of OCRL1 function leads to disease is ill defined. Here, we show that OCRL1 is associated with clathrin-coated transport intermediates operating between the trans-Golgi network (TGN) and endosomes. OCRL1 interacts directly with clathrin heavy chain and promotes clathrin assembly in vitro. Interaction with clathrin is not, however, required for membrane association of OCRL1. Overexpression of OCRL1 results in redistribution of clathrin and the cation-independent mannose 6-phosphate receptor (CI-MPR) to enlarged endosomal structures that are defective in retrograde trafficking to the TGN. Depletion of cellular OCRL1 also causes partial redistribution of a CI-MPR reporter to early endosomes. These findings suggest a role for OCRL1 in clathrin-mediated trafficking of proteins from endosomes to the TGN and that defects in this pathway might contribute to the Lowe syndrome phenotype.

Animals↗
PubMed2005

Convergence of cell cycle regulation and growth factor signals on GRASP65.

Together with other Golgi matrix components, GRASP65 contributes to the stacking of Golgi cisternae in interphase cells. During mitosis, GRASP65 is heavily phosphorylated, and in turn, cisternal stacking is inhibited leading to the breakdown of the Golgi apparatus. Here we show that GRASP65 is phosphorylated on serine 277 in interphase cells, and this is strongly enhanced in response to the addition of serum or epidermal growth factor. This is directly mediated by ERK suggesting that GRASP65 has some role in growth factor signal transduction. Phosphorylation of Ser-277 is also dramatically increased during mitosis, however this is mediated by Cdk1 and not by ERK. The microinjection of recombinant GRASP65 without N-terminal myristoylation or a peptide fragment containing Ser-277 into the cytosol of normal rat kidney cells inhibits passage through mitosis. This effect is abolished when Ser-277 is replaced with alanine suggesting the phosphorylation of Ser-277 plays an important role in cell cycle regulation. The convergence of cell cycle regulation and growth factor signals on GRASP65 Ser-277 suggests that GRASP65 may function as a signal integrator controlling the cell growth.

Amino Acid Sequence↗
PubMed2005

Phosphoinositides and membrane traffic at the trans-Golgi network.

Cargo proteins moving along the secretory pathway are sorted at the TGN (trans-Golgi network) into distinct carriers for delivery to the plasma membrane or endosomes. Recent studies in yeast and mammals have shown that formation of these carriers is regulated by PtdIns(4)P. This phosphoinositide is abundant at the TGN and acts to recruit components required for carrier formation to the membrane. Other phosphoinositides are also present on the TGN, but the extent to which they regulate trafficking is less clear. Further characterization of phosphoinositide kinases and phosphatases together with identification of new TGN-associated phosphoinositide-binding proteins will reveal the extent to which different phosphoinositides regulate TGN trafficking, and help define the molecular mechanisms involved.

Animals↗
PubMed2005

Structure and function of the Lowe syndrome protein OCRL1.

Oculocerebrorenal syndrome of Lowe (OCRL) is an X-linked disorder with the hallmark features of congenital cataracts, mental retardation and Fanconi syndrome of the kidney proximal tubules. OCRL was first described in 1952, and exactly four decades later, the gene responsible was identified and found to encode a protein highly homologous to inositol polyphosphate 5-phosphatase. This suggested that Lowe syndrome may represent an inborn error of inositol phosphate metabolism, and subsequent studies confirmed that such metabolism is indeed perturbed in Lowe syndrome cells. However, the mechanism by which loss of function of the OCRL1 protein brings about Lowe syndrome remains ill defined. In this review, I will discuss our understanding of OCRL1, including where it is localized, what it interacts with and what its possible functions might be. I will then discuss possible mechanisms by which loss of OCRL1 may bring about cellular defects that manifest themselves in the pathology of Lowe syndrome.

Actins↗
PubMed2004

Spectrally resolved femtosecond two-color three-pulse photon echoes: study of ground and excited state dynamics in molecules.

We report the use of spectrally resolved femtosecond two-color three-pulse photon echoes as a potentially powerful multidimensional technique for studying vibrational and electronic dynamics in complex molecules. The wavelengths of the pump and probe laser pulses are found to have a dramatic effect on the spectrum of the photon echo signal and can be chosen to select different sets of energy levels in the vibrational manifold, allowing a study of the dynamics and vibrational splitting in either the ground or the excited state. The technique is applied to studies of the dynamics of vibrational electronic states in the dye molecule Rhodamine 101 in methanol.

Journal Article↗
PubMed2004

Caspase-mediated cleavage of syntaxin 5 and giantin accompanies inhibition of secretory traffic during apoptosis.

We report the caspase-dependent cleavage of two Golgi-associated transport factors during apoptosis. The tethering factor giantin is rapidly cleaved both in vitro and in vivo at a conserved site, to generate a stable membrane-anchored domain and a soluble domain that is subject to further caspase-dependent cleavage. The t-SNARE syntaxin 5 is also cleaved rapidly, resulting in the separation of the catalytic membrane-proximal domain from an N-terminal regulatory domain. Cleavage of giantin and syntaxin 5 is accompanied by a cessation of vesicular transport between the ER and the Golgi complex, which first manifests itself as a block in ER exit. The contribution that such an inhibition of trafficking may make towards the generation of an apoptotic phenotype is discussed.

Animals↗
PubMed2003

The coiled-coil membrane protein golgin-84 is a novel rab effector required for Golgi ribbon formation.

Fragmentation of the mammalian Golgi apparatus during mitosis requires the phosphorylation of a specific subset of Golgi-associated proteins. We have used a biochemical approach to characterize these proteins and report here the identification of golgin-84 as a novel mitotic target. Using cryoelectron microscopy we could localize golgin-84 to the cis-Golgi network and found that it is enriched on tubules emanating from the lateral edges of, and often connecting, Golgi stacks. Golgin-84 binds to active rab1 but not cis-Golgi matrix proteins. Overexpression or depletion of golgin-84 results in fragmentation of the Golgi ribbon. Strikingly, the Golgi ribbon is converted into mini-stacks constituting only approximately 25% of the volume of a normal Golgi apparatus upon golgin-84 depletion. These mini-stacks are able to carry out protein transport, though with reduced efficiency compared with a normal Golgi apparatus. Our results suggest that golgin-84 plays a key role in the assembly and maintenance of the Golgi ribbon in mammalian cells.

Animals↗
PubMed2002

Golgi complex: biogenesis de novo?

Whether Golgi biogenesis occurs by self-assembly or around a pre-existing template is currently a matter of debate. Recent studies have shown that Golgi structural proteins are more dynamic than previously thought, suggesting that self-assembly of the Golgi complex may be possible.

Animals↗
PubMed2002

Caspase-mediated cleavage of the stacking protein GRASP65 is required for Golgi fragmentation during apoptosis.

The mammalian Golgi complex is comprised of a ribbon of stacked cisternal membranes often located in the pericentriolar region of the cell. Here, we report that during apoptosis the Golgi ribbon is fragmented into dispersed clusters of tubulo-vesicular membranes. We have found that fragmentation is caspase dependent and identified GRASP65 (Golgi reassembly and stacking protein of 65 kD) as a novel caspase substrate. GRASP65 is cleaved specifically by caspase-3 at conserved sites in its membrane distal COOH terminus at an early stage of the execution phase. Expression of a caspase-resistant form of GRASP65 partially preserved cisternal stacking and inhibited breakdown of the Golgi ribbon in apoptotic cells. Our results suggest that GRASP65 is an important structural component required for maintenance of Golgi apparatus integrity.

Amino Acid Chloromethyl Ketones↗
PubMed2002

Membranes and motors.

The 55(th) Harden Conference on the dynamics of membrane traffic convened on the 25(th) August 2002 in the English Lake District. This meeting, which was organized by Bob Burgoyne and Viki Allan under the auspices of the Biochemical Society, comprised a highly enjoyable mixture of well-presented talks with uncommonly good weather. Presentations covered a broad range of topics relating to the dynamics of protein transport along both the exocytic and endocytic pathways of eukaryotic cells.

Animals↗