Proteins organelles associated with

Rab is a family of small G proteins involved in membrane vesicle trafficking. Mammalian tissues contain around 30 forms of Rab, which specifically associate with the various types of membrane vesicles and organelles that exist in cells [30,31]. Rab proteins, named originally as ras-related proteins in brain, are isoprenylated and associate with membranes, as do isoprenylated Ras and G protein y subunits. However, unlike these other G proteins, the GTP and GDP binding to Rab appears to regulate its association with membrane compartments. 

Substrate availability to the cell is affected by the supply of raw materials from the environment. The plasma membranes of cells incorporate special and often specific transport proteins (translocases) or pores that permit the entry of substrates into the cell interior. Furthermore pathways in eukaryotic cells are often compartmentalized within cytoplasmic organelles by intracellular membranes. Thus we find particular pathways associated with the mitochondria, the lysosomes, the peroxisomes, the endoplasmic reticulum for example. Substrate utilization is limited therefore by its localization at the site of need within the cell and a particular substrate will be effectively concentrated within a particular organelle. The existence of membrane transport mechanisms is crucial in substrate delivery to, and availability at, the site of use. 

---