Cells have to maintain stable plasma membrane protein and lipid compositions under normal conditions and to remodel their plasma membranes in response to stimuli. This maintenance and remodeling require that integral membrane proteins at the plasma membrane that become misfolded, due to the relatively harsher extracellular milieu or carbohydrate and amino acid sequence changes, are degraded. We had previously shown that Derlin proteins, required for quality control mechanisms in the Endoplasmic Reticulum, also localize to endosomes and function in the degradation of misfolded integral membrane proteins at the plasma membrane. In this study, we show that Derlin proteins physically associate with Sorting Nexins that function in retrograde membrane transport from endosomes to the Golgi apparatus. Using genetic studies in Caenorhabditis elegans and ricin pulse-chase analyses in murine RAW264.7 macrophages, we show that the Derlin-Sorting Nexin interaction is physiologically relevant. Our studies suggest that at least some integral membrane proteins that are misfolded at the plasma membrane are retrogradely transported to the Golgi apparatus and ultimately to the Endoplasmic Reticulum for degradation via resident quality control mechanisms.

Mucolipidosis type IV is a lysosomal storage disorder resulting from mutations in the MCOLN1 gene, which encodes the endosomal/lysosomal Transient Receptor Potential channel protein mucolipin-1/TRPML1. Cells isolated from Mucolipidosis type IV patients and grown in vitro and in in vivo models of this disease both show several lysosome-associated defects. However, it is still unclear how TRPML1 regulates the transport steps implicated by these defects. Identifying proteins that associate with TRPML1 will facilitate the elucidation of its cellular and biochemical functions. We report here two saturation screens for proteins that interact with TRPML1: one that is based on immunoprecipitation/mass spectrometry and the other using a genetic yeast two-hybrid approach. From these screens, we identified largely non-overlapping proteins, which represent potential TRPML1-interactors., Using additional interaction assays on some of the potential interactors from each screen, we validated some proteins as candidate TRPML1 interactors In addition, our analysis indicates that each of the two screens not only identified some false-positive interactors, as expected from any screen, but also failed to uncover potential TRPML1 interactors. Future studies on the true interactors, first identified in these screens, will help elucidate the structure and function of protein complexes containing TRPML1.

This report describes a student-run psychiatry clinic with a dual mission of education and service, and the challenges associated with these sometimes competing goals. This clinic serves a vital need within our community and may be an example of the role that student-run clinics can have in fostering interdisciplinary care, psychiatric recruitment, and training for medical students.

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