Le réticulum endoplasmique (RE) est le premier compartiment intracellulaire traversé par la voie de sécrétion des protéines. Au sein de cet organite, les protéines destinées à la sécrétion, à la membrane ou les protéines résidentes de la voie de sécrétion acquièrent une conformation native, et subissent une multitude de modifications post-traductionnelles incluant la glycosylation et la formation de ponts disulfures intra et intermoléculaires. Généralement, seules les protéines bien conformées sont véhiculées hors du RE grâce à un système de surveillance très fin dont le rôle est de vérifier la conformation correcte des protéines, de retenir les protéines mal conformées jusqu’à ce qu’elles atteignent une conformation adéquate ou de les adresser à la dégradation. Malgré le nombre important de protéines traversant la voie de sécrétion et de pathologies affectant le processus de repliement, seul un petit nombre de protéines ont été décrites dans le système de surveillance du RE et le système de repliement/dégradation qui lui est associé. Au cours de ma thèse, j’ai participé à la mise en place d’une stratégie expérimentale permettant d’isoler des facteurs impliqués dans le système de surveillance du repliement des protéines. Cette méthodologie nous a permis d’identifier et de caractériser la protéine AGR2 (Anterior Gradient 2), un membre putatif de la famille des Protein Disulfide Isomerase (PDI) qui semble interagir très précocement avec les polypeptides en voie de synthèse. Les résultats de notre étude fonctionnelle suggèrent qu’AGR2 joue un rôle dans l’homéostasie basale du RE et intervient spécifiquement dans la régulation de la dégradation des protéines mal conformées. L’étude du rôle moléculaire ce nouvel acteur du système de surveillance du RE pourrait permettre de progresser dans la compréhension de ce processus crucial pour l’homéostasie cellulaire. / The Endoplasmic Reticulum (ER) is the first intracellular compartment encountered by secretory proteins. In this organelle, secretory proteins and integral membrane proteins acquire their correct conformation and undergo many post-translational modifications such as N-glycosylation or disulfide bond formation. To ensure that proteins are properly folded, the ER has evolved a quality control system achieving surveillance on the protein folding status. Partially folded or misfolded proteins are not allowed to escape this compartment and remain in the ER or are taken in charge by the degradation machinery. About one third of the genome products mature in the ER, however thus far an apparently small number of quality control actors have been identified as being responsible for the survey of these proteins’s conformation. The importance of understanding principles regulating this mechanism is highlighted by the number of protein-misfolding human diseases. During my PhD thesis, I participate to a study aiming at discovering novel constituents of the ER quality control system. Our approach led to the identification and the characterization of Anterior Gradient 2 protein (AGR2) a potential member of the PDI-like family that interacts with nascent translocating polypeptides. The results of a functional study show that AGR2 is implicated in basal ER homeostasis and participates in the quality control capacity of the ER by specifically regulating the degradation of terminally misfolded proteins.

The Anterior Gradient 2 (AGR2) protein has been implicated in a variety of biological systems linked to cancer and metastasis, tamoxifen-induced drug resistance, pro-inflammatory diseases like IBD and asthma, and limb regeneration. The molecular mechanisms by which AGR2 mediates these various phenotypes in disease progression in both cancer and IBD are poorly understood, as is the biological function(s) of AGR2 under non-disease conditions. Here, we use a combination of biochemical techniques, organ culture, cell biology and mouse genetics to investigate the biological significance of AGR2 both in cell lines and in vivo. We present data based on phage-peptide inter-actomics screens suggesting a role for AGR2 in mediating the maturation and trafficking of a class of membrane and secretory proteins, and investigate a putative interaction between AGR2 and one member of this class of proteins. We also describe the construction of a universal vector for use in making a variety of transgenic animals, and then present data showing its use as a promoter reporter, and attempt to investigate the temporal and spatial expression of AGR2 in the developing and adult mouse. Further, we present data describing the localisation pattern of AGR2 in the developing murine kidney using a combination of organ culture and antibody staining, and suggest a role for AGR2 in the developing kidney based on this data that is in agreement with a chaperone function for membrane and secretory proteins. Together, these data suggest that AGR2 has an intrinsic consensus docking site for a subset of its client proteins, that AGR2 plays a role in protein maturation in ciliated cell types, and provides a novel biological model to dissect the role of AGR2 in ER-trafficking.