The Golgi Cytoskeleton

Control of the Golgi cytoskeleton by G-proteins.

The Rab family of GTPase proteins (Martinez & Goud, 1998) are known to control membrane traffic mediated through guanine-nucleotide exchange factors (GEFs) (Stow & Heimann 1998; Ullrich et al, 1994).

The Golgi apparatus disassembles at mitosis and reassembles afterwards.  This offers an opportunity to study the natural assembly process.  Phosphorylation is an important key and is as one may expect, orchestrated by many of the same kinases that direct many of the other steps in mitosis  such as spindle pole formation, nuclear membrane breakdown and cytokinesis.  The distinctive shape of the Golgi as a series of plate-like structures is due to proteins whose function it is to bridge the gap between the stacks and to lock them together.  One such protein has been identified from Golgi membrane is called "GRASP65" (Golgi Re-Assembly Stacking Protein that is 65 kDa in size (Barr et al, 1997).  GRASP65 interacts with another protein GM130.  GRASP65 is a myristoylated peripheral membrane protein and must bind to yet another partner to explain its tropism to Golgi membranes.  GM130 is unlikely to do this as it has no detectable membrane-binding attributes (membrane spanning domains or fatty acid modification). However, GM130 is known to bind to p115, a protein known to be involved in vesicle docking (Sapperstein et al, 1996) so this may well be the Gogli specific targeting mechanism. GRASP65 is essential for Golgi stacking and is phosphorylated at mitosis (as is GM130), by mitotic kinase, this phosphorylation inhibits its interaction with p115 through GM130.  

Another Golgi-specific complex is formed by golgin-245/p230 and golgin-97.  Each of these proteins contains a so called GRIP domain which recruits coiled-coil proteins to the Golgi (Munro & Nichols, 1999).

It is clear that the membranous structures that form the Golgi apparatus have a constellation of ABPs and other proteins but what makes a Golgi membrane uniquely Golgi in the first place? The Golgi is a specialized extension of the endoplasmic reticulum and trafficking from the two system occurs constantly.