Practice question

Movement of cargo through Golgi stack, comprising cis, medial and trans cisternae plus trans-Golgi network, originally debated between stable cisternae with anterograde vesicle shuttling versus progressive maturation. Current consensus supported by live imaging in Saccharomyces cerevisiae where individual Golgi cisternae are dispersed and resolvable, by quantitative immunoelectron tomography and super-resolution microscopy, favors cisternal maturation model as primary mechanism for anterograde transport. New cis cisternae form de novo at ER-Golgi interface through homotypic fusion of COPII-derived carriers and ERGIC elements, supplied with cargo. Cisternae then move toward trans face while maturing by exchanging resident enzymes via retrograde COPI vesicles that recycle cis glycosyltransferases backward, converting cis to medial to trans identity, while cargo remains inside lumen. At trans-Golgi network, clathrin and adaptors sort lysosomal hydrolases to endosomes and secretory cargo to plasma membrane. Model naturally accommodates large supramolecular cargo such as procollagen triple helices three hundred nanometers long, algal scales and casein micelles that cannot fit into sixty nanometer vesicles, explains transient cisternal composition changes and integrates vesicular transport as retrograde recycling rather than forward shuttling, harmonizing biochemical and morphological observations across species and secretory systems.