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Sean Munro

Publications and source records attributed to Sean Munro.

23 records · Page 2Linked to original sources
PubMed2002

Vesicle tethering complexes in membrane traffic.

Despite the recent progress in the field of membrane traffic, the question of how the specificity of membrane fusion is achieved has yet to be resolved. It has become apparent that the SNARE proteins, although central to the process of fusion, are often not the first point of contact between a vesicle and its target. Instead, a poorly understood tethering process physically links the two before fusion occurs. Many factors that have an apparent role in tethering have been identified. Among these are several large protein complexes. Until recently, these seemed unrelated, which was a surprise since proteins involved in membrane traffic often form families, members of which function in each transport step. Recent work has shown that three of the complexes are in fact related. We refer to these as the 'quatrefoil' tethering complexes, since they appear to share a fourfold nature. Here we describe the quatrefoil complexes and other, unrelated, tethering complexes, and discuss ideas about their function. We propose that vesicle tethering may have separate kinetic and thermodynamic elements and that it may be usefully divided into events upstream and downstream of the function of Rab GTPases. Moreover, the diversity of tethering complexes in the cell suggests that not all tethering events occur through the same mechanisms.

Animals↗
PubMed2002

Targeting of Golgi-specific pleckstrin homology domains involves both PtdIns 4-kinase-dependent and -independent components.

BACKGROUND: Phosphoinositides are required for the recruitment of many proteins to both the plasma membrane and the endosome; however, their role in protein targeting to other organelles is less clear. The pleckstrin homology (PH) domains of oxysterol binding protein (OSBP) and its relatives have been shown to bind to the Golgi apparatus in yeast and mammalian cells. Previous in vitro binding studies identified phosphatidylinositol (PtdIns) (4)P and PtdIns(4,5)P(2) as candidate ligands, but it is not known which is recognized in vivo and whether phosphoinositide specificity can account for Golgi-specific targeting. RESULTS: We have examined the distribution of GFP fusions to the PH domain of OSBP and to related PH domains in yeast strains carrying mutations in individual phosphoinositide kinases. We find that Golgi targeting requires the activity of the PtdIns 4-kinase Pik1p but not phosphorylation of PtdIns at the 3 or 5 positions and that a PH domain specific for PtdIns(4,5)P(2) is targeted exclusively to the plasma membrane. However, a mutant version of the OSBP PH domain that does not bind phosphoinositides in vitro still shows some targeting in vivo. This targeting is independent of Pik1p but dependent on the Golgi GTPase Arf1p. CONCLUSIONS: Phosphorylation of PtdIns at the 4 position but not conversion to PtdIns(4,5)P(2) contributes to recruitment of PH domains to the Golgi apparatus. However, potential phosphoinositide ligands for these PH domains are not restricted to the Golgi, and the OSBP PH domain also recognizes a second determinant that is ARF dependent, indicating that organelle specificity reflects a combinatorial interaction.

1-Phosphatidylinositol 4-Kinase↗
PubMed2002

Organelle identity and the targeting of peripheral membrane proteins.

Within the secretory pathway, most proteins involved in vesicle formation, motor recruitment and vesicle tethering are not integral membrane proteins but, rather, peripheral membrane proteins recruited to the relevant organelles from the cytosol. From recent studies on diverse organelles, it appears that such recruitment is usually mediated by binding to a labile determinant, such as an activated G protein or a short-lived lipid species, whose distribution is restricted to a single organelle. This suggests that these determinants are what specify organelle identity, and raises interesting questions about how they are generated in an organelle-specific fashion.

Animals↗
PubMed2002

CASP, the alternatively spliced product of the gene encoding the CCAAT-displacement protein transcription factor, is a Golgi membrane protein related to giantin.

Large coiled-coil proteins are being found in increasing numbers on the membranes of the Golgi apparatus and have been proposed to function in tethering of transport vesicles and in the organization of the Golgi stack. Members of one class of Golgi coiled-coil protein, comprising giantin and golgin-84, are anchored to the bilayer by a single C-terminal transmembrane domain (TMD). In this article, we report the characterization of another mammalian coiled-coil protein, CASP, that was originally identified as an alternatively spliced product of the CUTL1 gene that encodes CCAAT-displacement protein (CDP), the human homologue of the Drosophila homeodomain protein Cut. We find that the Caenorhabditis elegans homologues of CDP and CASP are also generated from a single gene. CASP lacks the DNA binding motifs of CDP and was previously reported to be a nuclear protein. Herein, we show that it is in fact a Golgi protein with a C-terminal TMD and shares with giantin and golgin-84 a conserved histidine in its TMD. However, unlike these proteins, CASP has a homologue in Saccharomyces cerevisiae, which we call COY1. Deletion of COY1 does not affect viability, but strikingly restores normal growth to cells lacking the Golgi soluble N-ethylmaleimide-sensitive factor attachment protein receptor Gos1p. The conserved histidine is necessary for Coy1p's activity in cells lacking Gos1p, suggesting that the TMD of these transmembrane Golgi coiled-coil proteins is directly involved in their function.

Alternative Splicing↗