GMx33: A Novel Family of trans-Golgi Proteins Identified by Proteomics

Abstract

The known functions of the Golgi complex include the sorting, packaging, post-translational modification, and transport of secretory proteins, membrane proteins, and lipids. Other functions still remain elusive to cell biologists. With the goal of identifying novel Golgi proteins, a proteomics project was undertaken to map the major proteins of the organelle using two-dimensional gels, to identify the unknowns using tandem mass spectrometry, and to screen for Golgi residents using GFP-fusion constructs. Multiple unknowns were identified, and the initial characterization of one of these proteins is reported here. GMx33 alpha is a member of a conserved family of cytosolic Golgi-associated proteins with no known homology to any known functional domain or protein. Biochemical analyses show that GMx33 alpha differentially partitions into all phases of multiple detergent extractions, and two-dimensional immunoblots reveal that there are multiple differentially modified forms of GMx33 alpha associated with the Golgi, several of which are phosphorylated. Evidence suggests that these post-translational modifications regulate its association with the Golgi. GMx33 alpha was not found on Golgi budded vesicles, and immuno-electron microscopy co-localizes GMx33 alpha to the trans-face on the same three cisternae as TGN38 in normal rat kidney cells. This work represents the preliminary characterization of a novel family of trans-Golgi-associated proteins.

Role of Phosphatidylinositol 4-Phosphate (PI4P) and Its Binding Protein GOLPH3 in Hepatitis C Virus Secretion

“…Given that TGN-localized PI4P and several of its binding proteins have already been implicated in HCV secretion (34,35), we sought to further characterize the role of a newly identified PI4P binding Golgi protein, GOLPH3, in the HCV secretion process (34,35). GOLPH3 is a Golgi-associated protein and a functional component of Golgi secretory pathway (37,56). GOLPH3 localizes to the Golgi via its interaction with PI4P and maintains Golgi morphology by interacting with an unconventional myosin, MYO18A, which in turn binds to F-actin.…”

GOLPH3, a Good Prognostic Indicator in Early-stage NSCLC Related to Tumor Angiogenesis

“…Golgi phosphoprotein-3 (GOLPH3), also known as GPP34/GMx33/MIDAS, is a highly conserved protein with molecular weight of 34 kDa, which plays a role in anterograde and retrograde Golgi trafficking (Wu et al, 2000;Nakashima et al, 2005;Snyder et al, 2006). GOLPH3 localizes in human chromosome 5p13, a region that is requently amplified in multiple solid tumor types (Bohm et al, 2002;Yokoi et al, 2002;Gorringe et al, 2005).…”

Structure of the Golgi and Distribution of Reporter Molecules at 20°C Reveals the Complexity of the Exit Compartments

“…To ask whether all Golgi proteins are similarly degraded, we extended the immunoblot analysis to include two additional Golgi markers. Remarkably, the cationic independent mannose-6-phosphate receptor (CI-MPR), another transmembrane protein predominately localized to the trans-Golgi (Griffiths et al, 1988), and GmX33, a trans-matrix protein (Wu et al, 2000) remained relatively constant, suggesting the degradation is selective. The steady state amount of both TGN38 and MG160 recovered within 2 h, demonstrating that the block is reversible.…”

“…It may be involved in modifying the lipid composition of trans-cisternal membranes for sorting into lipid rafts or "sphingolipid-cholesterol rich microdomains" (Simons and Ikonen, 1997;Brown and London, 2000). Additionally, the specialized ER may be involved in modifying the Golgi matrix to facilitate cisternal maturation (Wu et al, 2000;Marsh et al, 2001b). Perhaps the specialized ER, as it adheres to trans-cisternae, contributes enzymes that result in disassociation of the cisternae from the stack to facilitate the exit processes.…”