Identification of a Novel Post-ER, Pre-Golgi Compartment Where Unassembled Monomers of Oligomeric Proteins Accumulate
It has been clear for some time that newly synthesized proteins exit the ER via vesicular carriers that are derived from transitional or part rough/part smooth elements of the ER and pass through pre-Golgi elements before reaching the cis side of the Golgi stack (Farquhar, 1985; Farquhar, 1991; Pfeffer and Rothman, 1987). What is not so clear is the number and nature of the pre-Golgi compartments that are variously referred to as pre-Golgi, intermediate or salvage compartments. The existence of an intermediate compartment was originally suggested by Saraste and Kuismanen (1984) based on their observation that when cells infected with Semiliki Forest virus were incubated at 15°C, the envelope glycoproteins accumulated in smooth vacuoles located near the cis side of the Golgi. It was also shown that some viruses bud from smooth ER, pre-Golgi compartments (Tooze et al., 1984; Ulmer and Palade, 1991). Pelham (1989) proposed that one of the functions of this intermediate compartment is to act as a salvage or recycling compartment where resident ER proteins that escape from the ER are specifically and continuously retrieved and returned to the ER by a receptor that recognizes KDEL or closely related sequences. Pelham and co-workers also obtained experimental evidence indicating that this intermediate compartment may contain N-acetyl glucosamine-1 phosphotransferase which is responsible for addition of the phosphorylated glcNAc to lysosomal enzymes (Pelham, 1989). Moreover, three proteins—i.e., p53 (Schweizer et al., 1988), p58 (Hendricks et al., 1991; Saraste et al., 1987), and p63 (Schweizer et al., 1991), have been identified that represent putative markers for the intermediate compartment with the complication that they appear not to be limited to this compartment, but to cycle between the intermediate compartment and the cis Golgi (Saraste and Svensson, 1991). Using the 53 kD protein as a marker, Schweizer and co-workers (Schweizer et al., 1991) have prepared subfractions enriched in this protein. These fractions were largely separated from rough ER proteins (ribophorins, BIP, and PDI) as well as from N-acetyl glucosamine-1 phosphotransferase and phosphodiesterase. For the purposes of our discussion it is important to realize that a certain amount of confusion exists concerning the number and nature of the putative intermediate compartments and their relationship to the Golgi.
KeywordsProtein Disulfide Isomerase Vesicular Stomatitis Virus Envelope Glycoprotein Rubella Virus Tubular Membrane
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