Models of Golgi Maturation

The Golgi apparatus is a cellular organelle found within all nucleated mammalian cells. The Golgi serves as an intermediate vesicle for proteins which are being processed in order to be exported from teh cell or moved to other parts of the cell. Despite having been discovered decades ago, it is still not known how the Golgi apparatus truly functions, and two main models of Golgi apparatus maturation have emerged over time, both with some evidence to support them. These models are the cisternal maturation model and the vesicular transport model, and both should be carefully considered by anyone looking to perform an experiment involving this compartment.

Assumptions of Each Model

The cisternal maturation model predicts that Golgi cargo proteins will remain associated with a single compartment of the Golgi apparatus while Golgi resident proteins continuously undergo COPI-mediated retrograde transport to the appropriate Golgi cisternae. A novel way to test the accuracy of this model would by means of yeast expressing a CFP-tagged medial Golgi resident protein and a YFP-tagged cargo protein. As yeast do not have stacked Golgi compartments, immunofluorescence microscopy can be used to track the CFP/YFP fluorescence in individual compartments. It is worth noting that this experiment would benefit from keeping expression of the YFP-cargo protein under experimental control of an inducible promoter, as if it were constitutively expressed then all cisternae would have fairly constant levels of YFP regardless of which model is accurate; the following predictions assume that the protein is under such control.

If the cisternal maturation model is correct then in this experimental system you would observe relatively constant levels of YFP within a given cisterna, as the cargo protein should remain in that cisterna for the duration of its maturation. In contrast, the level of CFP within a given cisterna should initially be low, increase as the cisterna matures into the medial Golgi, and should again decrease as the cisterna matures and the CFP-resident protein undergoes retrograde transport to the medial Golgi.

Alternatively, if the vesicular transport model were correct then within a given cisterna the level of CFP should remain constant as resident medial Golgi proteins should remain associated with medial Golgi cisternae rather than undergoing transport between cisternae. In contrast, YFP levels within a given cisterna should rise as the protein is transported into this cisterna, and should fall as it is transported to subsequent cisternae.

CFP and YFP undergo Förster resonance energy transfer (FRET) such that when the two fluorophores are closely associated (within ~10nm) excitation of CFP will lead to emission of light at a wavelength sufficient to excite YFP. If the cisternal maturation model is correct and a CFP-medial Golgi resident protein and a YFP-cargo protein are expressed in mammalian cells containing stacked Golgi, the resolution limit of light microscopes would make interpretation of data difficult. If cells were excited at the excitation wavelength of CFP and imaged at the emission wavelength of YFP, initially there would be low levels of signal immediately after induction of the YFP-cargo protein as the protein is synthesized and moves through the ER. As the protein reaches the Golgi signal will remain low until it is within ~10nm of the medial Golgi, at which point FRET will occur and the YFP signal will visibly increase before dropping off again as the cisternae mature and the CFP-resident protein is removed by means of retrograde transport. Unfortunately, due to the resolution limit it would not be possible to resolve individual cisternae, and thus FRET would indicate when the two proteins are both in the medial Golgi but would be unable to effectively distinguish between the two models of Golgi transport.

A Unified Model

While there is evidence supporting each of these models, there are certain results which they individually fail to explain. For example, a study using collagen aggregates demonstrated that these aggregates were too large to fit in standard COPI vesicles and yet they were still able to traverse the Golgi network, which is inconsistent with the vesicular transport model. There is also evidence that cargo-containing COPI vesicles devoid of Golgi resident enzymes exist at the trans Golgi, which cannot be reasonably explained by the cisternal maturation model. A unified model which combines aspects of both of these models may be the most likely explanation, as it helps to resolve the abovementioned inconsistencies.

According to the unified model, cisternae mature over time and COPI vesicles mediate retrograde flow of Golgi resident proteins, while other COPI vesicles which use different adaptor proteins mediate the anterograde flow of cargo proteins, serving as a faster route for protein processing provided the proteins are able to fit within these vesicles. This model would explain why collagen aggregates are able to cross the Golgi and yet they do so at a slower rate than most smaller proteins, which are able to make use of COPI-mediated rapid anterograde transport. This model would further predict that certain classes of COPI vesicles would contain primarily Golgi resident proteins whereas others would contain primarily cargo proteins, which is consistent with current research.


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