Editing Golgi apparatus (section)

== Current models of vesicular transport and trafficking ==
{{unsolved|biology|In cell theory, what is the exact transport mechanism by which proteins travel through the Golgi apparatus?}}
=== Model 1: Anterograde vesicular transport between stable compartments ===
* In this model, the Golgi is viewed as a set of stable compartments that work together. Each compartment has a unique collection of enzymes that work to modify protein cargo. Proteins are delivered from the ER to the ''cis'' face using [[COPII]]-coated vesicles. Cargo then progress toward the ''trans'' face in [[COPI]]-coated vesicles. This model proposes that COPI vesicles move in two directions: [[Axoplasmic transport#Anterograde transport|anterograde]] vesicles carry [[secretory protein]]s, while [[Axoplasmic transport#Retrograde transport|retrograde]] vesicles recycle Golgi-specific trafficking proteins.<ref name="Glick-2011">{{cite journal | vauthors = Glick BS, Luini A | title = Models for Golgi traffic: a critical assessment | journal = Cold Spring Harbor Perspectives in Biology | volume = 3 | issue = 11 | pages = a005215 | date = November 2011 | pmid = 21875986 | pmc = 3220355 | doi = 10.1101/cshperspect.a005215 }}</ref>
** '''Strengths:''' The model explains observations of compartments, polarized distribution of enzymes, and waves of moving vesicles. It also attempts to explain how Golgi-specific enzymes are recycled.<ref name="Glick-2011"/>
** '''Weaknesses:''' Since the amount of COPI vesicles varies drastically among types of cells, this model cannot easily explain high trafficking activity within the Golgi for both small and large cargoes. Additionally, there is no convincing evidence that COPI vesicles move in both the anterograde and retrograde directions.<ref name="Glick-2011"/>
* This model was widely accepted from the early 1980s until the late 1990s.<ref name="Glick-2011"/>

===Model 2: Cisternal progression/maturation===
* In this model, the fusion of COPII vesicles from the ER begins the formation of the first ''cis''-cisterna of the Golgi stack, which progresses later to become mature TGN cisternae. Once matured, the TGN cisternae dissolve to become secretory vesicles. While this progression occurs, COPI vesicles continually recycle Golgi-specific proteins by delivery from older to younger cisternae. Different recycling patterns may account for the differing biochemistry throughout the Golgi stack. Thus, the compartments within the Golgi are seen as discrete kinetic stages of the maturing Golgi apparatus.<ref name="Glick-2011"/>
** '''Strengths:''' The model addresses the existence of Golgi compartments, as well as differing biochemistry within the cisternae, transport of large proteins, transient formation and disintegration of the cisternae, and retrograde mobility of native Golgi proteins, and it can account for the variability seen in the structures of the Golgi.<ref name="Glick-2011"/>
** '''Weaknesses:''' This model cannot easily explain the observation of fused Golgi networks, tubular connections among cisternae, and differing kinetics of secretory cargo exit.<ref name="Glick-2011"/>

===Model 3: Cisternal progression/maturation with heterotypic tubular transport===
* This model is an extension of the cisternal progression/maturation model. It incorporates the existence of tubular connections among the cisternae that form the Golgi ribbon, in which cisternae within a stack are linked. This model posits that the tubules are important for bidirectional traffic in the ER-Golgi system: they allow for fast anterograde traffic of small cargo and/or the retrograde traffic of native Golgi proteins.<ref name="Glick-2011"/><ref name="Wei-2010">{{cite journal |vauthors=Wei JH, Seemann J |title=Unraveling the Golgi ribbon |journal=Traffic |volume=11 |issue=11 |pages=1391–400 |date=November 2010 |pmid=21040294 |pmc=4221251 |doi=10.1111/j.1600-0854.2010.01114.x |url=}}</ref>
** '''Strengths:''' This model encompasses the strengths of the cisternal progression/maturation model that also explains rapid trafficking of cargo, and how native Golgi proteins can recycle independently of COPI vesicles.<ref name="Glick-2011"/>
** '''Weaknesses:''' This model cannot explain the transport kinetics of large protein cargo, such as [[collagen]]. Additionally, tubular connections are not prevalent in plant cells. The roles that these connections have can be attributed to a cell-specific specialization rather than a universal trait. If the membranes are continuous, that suggests the existence of mechanisms that preserve the unique biochemical gradients observed throughout the Golgi apparatus.<ref name="Glick-2011"/>

===Model 4: Rapid partitioning in a mixed Golgi===
* This rapid partitioning model is the most drastic alteration of the traditional vesicular trafficking point of view. Proponents of this model hypothesize that the Golgi works as a single unit, containing domains that function separately in the processing and export of protein cargo. Cargo from the ER move between these two domains, and randomly exit from any level of the Golgi to their final location. This model is supported by the observation that cargo exits the Golgi in a pattern best described by exponential kinetics. The existence of domains is supported by fluorescence microscopy data.<ref name="Glick-2011"/>
** '''Strengths:''' Notably, this model explains the exponential kinetics of cargo exit of both large and small proteins, whereas other models cannot.<ref name="Glick-2011"/>
** '''Weaknesses:''' This model cannot explain the transport kinetics of large protein cargo, such as collagen. This model falls short on explaining the observation of discrete compartments and polarized biochemistry of the Golgi cisternae. It also does not explain formation and disintegration of the Golgi network, nor the role of COPI vesicles.<ref name="Glick-2011"/>

===Model 5: Stable compartments as cisternal model progenitors===
* This is the most recent model. In this model, the Golgi is seen as a collection of stable compartments defined by [[Rab (G-protein)]] [[GTPase]]s.<ref name="Glick-2011"/>
** '''Strengths:''' This model is consistent with numerous observations and encompasses some of the strengths of the cisternal progression/maturation model. Additionally, what is known of the [[Rab GTPase]] roles in mammalian endosomes can help predict putative roles within the Golgi. This model is unique in that it can explain the observation of "megavesicle" transport intermediates.<ref name="Glick-2011"/>
** '''Weaknesses:''' This model does not explain morphological variations in the Golgi apparatus, nor define a role for COPI vesicles. This model does not apply well for plants, algae, and fungi in which individual Golgi stacks are observed (transfer of domains between stacks is not likely). Additionally, megavesicles are not established to be intra-Golgi transporters.<ref name="Glick-2011"/>

Though there are multiple models that attempt to explain vesicular traffic throughout the Golgi, no individual model can independently explain all observations of the Golgi apparatus. Currently, the cisternal progression/maturation model is the most accepted among scientists, accommodating many observations across eukaryotes. The other models are still important in framing questions and guiding future experimentation. Among the fundamental unanswered questions are the directionality of COPI vesicles and role of Rab GTPases in modulating protein cargo traffic.<ref name="Glick-2011"/>