Dynamic Organellar Maps: a Proteomic Microscope for Cell Biology

Protein subcellular localization is critical for protein function, and many cell biological processes involve movements of proteins between compartments. We have recently developed a mass-spectrometry based approach to determine the subcellular localization of thousands of proteins simultaneously. This multichannel ‘proteomic microscope’ yields organellar maps that give detailed insights into the organization of subcellular compartments. In addition, a comparison of maps made under different experimental conditions (for example drug treatments or genetic alterations) reveals which proteins change localization. Detection of such translocations allows functional deductions about cellular processes, such as membrane trafficking, and signalling (Figure 1). The approach is unbiased and systematic, making it ideal for the de novo investigation of poorly characterized processes and drug action.

Figure 1: Application of Dynamic Organellar Mapping reveals subcellular protein movements following stimulation with EGF (20 min, HeLa cells). Arrows indicate some of the detected compartment transitions; numbers show estimates of how many copies of a protein are moving in an average cell.

The AP-4 Pathway

Adaptor protein 4 (AP-4) is a tetrameric complex that mediates selective protein export form the trans-Golgi network (TGN). Mutations in any of the four genes encoding the AP-4 subunits cause severe intellectual disability and progressive spastic paraplegia. We are investigating the molecular details of AP-4 deficiency syndrome. We have recently uncovered a direct link between AP-4 mediated transport and the spatial control of autophagy, via sorting of the core autophagy machinery protein ATG9A, providing a potential mechanism for AP-4 pathology (Figure 2).

Figure 2: ATG9A trafficking is perturbed in fibroblasts from patients with mutations in AP-4 (shown here for the AP-4 beta subunit/SPG47). In the AP-4 mutant, ATG9A accumulates at the TGN, as evidenced by extensive co-localization with the marker protein TGN46.


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