Supporting Evidence for the Theory of Cisternal Maturation-Progression Abstract
Two competing theories have been postulated for explaining how proteins traverse the Golgi: cisternal maturation-progression and vesicular transport. Cisternal maturation-progression postulates that proteins move through the Golgi within cisternae which progress from cis to medial to trans. Large protein aggregates have been shown to move through the Golgi in a manner consistent with cisternal maturation-progression. COPI vesicles act as retrograde transport vehicles and have been shown to incorporate resident Golgi enzymes. A theory of competition among resident Golgi enzymes has been proposed to explain the polarity and specific distribution of enzymes throughout the Golgi, although further investigation is needed. Individual cisternae have been observed using time-lapse 3D microscopy to contain cis, medial, and trans resident Golgi enzymes progressively. Pulse-chase experiments have been coupled with the time-lapse 3D microscopy to show that cisternal maturation-progression can account for the rapid secretion of certain proteins. Further investigation is needed utilizing various cellular systems. Background
How do proteins traverse the Golgi apparatus? This question is the basis of a fundamental argument regarding the origin, development and function of the Golgi. The Golgi is an essential organelle that is intricately involved in the transport and modification of proteins. Most proteins are synthesized within the endoplasmic reticulum, then transported to and modified sequentially within the cis, medial and trans Golgi. This organelle is highly involved in both the endocytic and exocytic pathways and as such is essential for cell survival. There are two distinct theories that prevail for explaining how proteins move through the Golgi: the theory of vesicular transport and the theory of cisternal maturation-progression. Vesicular transport postulates that the cis, medial and trans Golgi are static entities and that proteins are transported forward from one region to the next via transport vesicles that bud from the rims of each static cisternae. In contrast, cisternal maturation-progression states that rather than being static entities, Golgi cisternae are dynamic structures that originate from vesicles in the endoplasmic reticulum, accumulate to form cis cisternae, then progress sequentially from cis to medial to trans cisternae and eventually the trans cisternae bud off into various transport vesicles. Currently both of these theories are supported by distinct groups of researchers and as such no consensus exists regarding protein trafficking through the Golgi apparatus. The purpose of this paper is to present supporting evidence for the theory of cisternal maturation-progression. Prior to delving into the specifics regarding vesicular transport and cisternal maturation-progression, it is important to establish the basic mechanisms associated with both of these theories. Coats are spherical protein shells with fixed compositions made up of multiple copies of cytosol derived subunits (coatomers) (Rothman & Wieland 1996). The two types of coated vesicles discussed in this review are COPI and COPII. The physical difference between COPI and COPII vesicles are the subunits that are included within each of these coats. COP coats are attached to budding membranes via the GTPase ADP-ribosylation factor 1 (ARF1). The COP coat is assembled when ARF1 is bound to GTP. When ARF1 hydrolyzes GTP to GDP the COP coat disassembles, thereby allowing the vesicle to dock with the corresponding acceptor membrane. Vesicle targeting and docking is mediated by a class of proteins known as Soluble NSF Attachment Protein Receptors (SNAREs). The vesicles contain v-SNAREs and the acceptor membranes or target membranes contain t-SNAREs. v-SNAREs are covered by the COP coat and therefore vesicles cannot attach to their target membranes until the...
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