The Golgi matrix is a collection of proteins involved in the structure and function of the Golgi apparatus. [1] [2] [3] The matrix was first isolated in 1994 as an amorphous collection of 12 proteins that remained associated together in the presence of detergent (which removed Golgi membranes) and 150 m M NaCl (which removed weakly associated proteins). [4] Treatment with a protease enzyme removed the matrix, which confirmed the importance of proteins for the matrix structure. [4] Modern freeze etch [5] electron microscopy (EM) clearly shows a mesh connecting Golgi cisternae and associated vesicles. [6] [7] Further support for the existence of a matrix comes from EM images showing that ribosomes are excluded from regions between and near Golgi cisternae. [8] [9] [10] [11] [12] [13]
The first individual protein component of the matrix was identified in 1995 as Golgin A2 (then called GM130). [14] Since then, many other golgin family proteins have been found to be in the Golgi matrix [2] and are associated with the Golgi membranes in a variety of ways. [15] [1] For example, GMAP210 (Golgi Microtubule Associated Protein 210) has an ALPS (Amphipathic Lipid-Packing Sensor) motif in the N-termal 38 amino acids and an ARF1-binding domain called GRAB (Grip-Related Arf-Binding) at the C-terminus. [16] Thus, the GRAB-domain can bind indirectly to Golgi cisternae and its ALPS motif can tether vesicles. [17] Golgins have coiled-coil domains and are thus predicted to have elongated structures [2] up to 200 nm in length. [18] Most are peripheral membrane proteins attached at one end to Golgi membranes. [2] They have flexible regions between the coiled-coil domains, which make them ideal candidates for mediating the dynamic vesicle docking to Golgi cisternae and dynamic structure of the Golgi itself. [2]
Golgi reassembly-stacking proteins are an evolutionarily conserved family of proteins in the Golgi matrix. [2] GRASP65 and GRASP55 are the 2 human GRASPs. These proteins were named from their requirement for accurate Golgi reassembly during an in vitro assay, [2] but they have also been shown to function in vivo, as shown in the accompanying figure. [19] GRASPs associate with lipid bilayers because they are myristoylated and their myristic acid residue intercalates into the lipid layer. [7] Their trans oligomerization is controlled by phosphorylation [6] and is thought to explain the fragmentation of the Golgi as required during mitosis. [7]
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