Chez les eucaryotes, les RNA Binding Proteins (RBP) s’associent aux transcrits pour former des Particules Ribo-Nucléoprotéiques (mRNP) dynamiques, dont la localisation et la composition sont déterminantes pour la maturation, l’export, la stabilité et la traduction des ARNm. Les protéines à motif LA sont des protéines de liaison à l’ARN, présentes chez plusieurs centaines d’espèces eucaryotes, qui se répartissent en 5 sous-familles : LA authentiques, LARP1, 4, 6 et 7. Les membres de ces sous-familles partagent des caractéristiques évolutives, des domaines additionnels et des fonctions conservés. Mes travaux de thèse ont contribué à l’étude fonctionnelle de protéines LARP6 chez Arabidopsis thaliana. On sait, à l’heure actuelle, que chez Manduca sexta et plusieurs espèces de vertébrés, LARP6 est impliquée dans la régulation de la différenciation cellulaire. Chez l’Homme, elle agit en tant que RBP pour coordonner la traduction des ARNm codant les sous-unités du collagène de type I. Les plantes vasculaires ont la particularité de contenir plusieurs protéines LARP6, classées en trois groupes évolutifs. Chez A. thaliana, l’unique représentant de chaque sous-famille semble s’être spécialisé. D’ailleurs, les protéines LARP6b et c ont des profils d’expression mutuellement exclusifs, où LARP6c est présente dans le pollen et LARP6b est ubiquitaire mais absente du gamétophyte mâle. Nous avons axé notre travail sur la protéine LARP6c et démontré qu’elle est cytoplasmique et impliquée, dans le pollen, dans le contrôle de la quantité d’ARNm codant des acteurs du transport vésiculaire. Les ARNm identifiés comme cibles potentielles de LARP6c codent, eux aussi, des facteurs impliqués dans le transport ; et possèdent dans leur 5’-UTR deux motifs qui pourraient permettre leur co-régulation par fixation de RBP. La délétion de LARP6c, affecte la capacité du tube pollinique à se diriger vers l’ovule suggérant un défaut de communication; ce qui est cohérent avec la dérégulation des ARNm codant des acteurs de la sécrétion/réception de signaux extracellulaires. Nous proposons que LARP6c intervient, dans le pollen, en tant que protéine de mRNP et co-régule la traduction et/ou la stabilité de transcrits codant des acteurs des voies de communications dépendantes de la sécrétion et de l’endocytose, et intervenant dans les échanges mâle/femelle / In eucaryotes, RNA Binding Proteins (RBP) associate with transcripts to form dynamic Ribo-Nucleoprotein Particles (mRNP), whose localization and composition are determinant for mRNA maturation, export, stability and translation. LA motif proteins are RNA binding proteins, found in several hundred eucaryotic species, which fall in 5 sub-families: genuine LA, LARP1, 4, 6 and 7. Members of these subfamilies share conserved evolutionary history, additional motifs and functions. My thesis work contributed to deciphering the functional properties of the Arabidopsis thaliana LARP6 proteins. Currently, we know that in Manduca sexta and many vertebrates species LARP6 is implicated in the regulation of cellular differentiation. In humans, it acts as an RBP to coordinate the translation of mRNA coding for type I collagen subunits. Vascular plants differ in possessing many LARP6 proteins classified in three evolutionary groups. In A. thaliana, the unique member of each subfamily seems to be specialized. LARP6b and c proteins present mutually exclusive expression profiles, with LARP6c only present in pollen and LARP6b ubiquitously expressed except in the male gametophyte. We mostly focused our work on LARP6c and showed it to be cytoplasmic and implicated in controlling the level of mRNAs encoding vesicular transport actors in pollen tubes. Putative identified LARP6c mRNA baits also encode proteins involved in transport and share two motifs in their 5’-UTR that could allow their co-regulation via RBP binding. LARP6c deletion induces deficiencies in pollen tube guidance towards the ovule, suggesting a communication default. This is consistent with the deregulation of mRNA coding for extra-cellular signal secretion/reception actors. We propose that LARP6c acts as an mRNP protein in pollen and co-regulates translation and/or stability of mRNA coding for actors of communication pathways depending on secretion and endocytosis; hence acting on male/female exchanges.

All cells carry the genetic machinery required to commit cell suicide; a process known as programmed cell death (PCD). While the ability to initiate PCD serves a number of useful purposes during development and homeostasis, misregulation of PCD is the underlying basis of most human diseases, including cancer, autoimmunity disorders and neurodegeneration. Using the tobacco hawkmoth Manduca sexta as a model organism, the Schwartz lab at UMass has demonstrated that PCD requires de novo gene expression and has cloned many death-associated genes. One of these genes encodes a novel protein that was named Acheron after one of the rivers of the Underworld in Greek mythology. Acheron (also known as Lupus Antigen Related Protein 6; Larp6) is an RNA binding protein that mediates a number of cellular processes, including cell survival, angiogenesis, migration, and differentiation. The molecular mechanisms that mediate Acheron’s diverse roles are poorly understood, but several lines of evidence suggest that it is mediated in part by protein protein interactions and motifs that target it to different cellular compartments. In this thesis, I employed immunofluorescence and confocal microscopy to conduct two studies employing mammalian cells. The first study was to determine the subcellular localization for Acheron in normal cells and cells treated with growth factors. I found that Acheron predominantly coincides with the microtubule cytoskeleton. In our second study, I tested the hypothesis that Acheron’s binding partners BAD, Human Homolog of Ariadne-1 (HHARI) and Calcium/Calmodulin-dependent Serine Protein Kinase (CASK) colocalize with Acheron. As part of this analysis, I sought to determine if Acheron could facilitate the translocation of CASK to the nucleus. I found that Acheron localizes predominantly to microtubules, with some expression in the cytoplasm and nucleus. When Acheron and its partners are co-labeled in cycling mouse C2C12 myoblasts or human U2-OS osteosarcoma cells, Acheron did not colocalize with BAD, HHARI or CASK on microtubules. However, I found that when Acheron is driven into the nucleus with high levels of growth factors, CASK also appeared to translocate to both the nucleus and to microtubules, where it colocalized with Acheron. The data acquired through these studies should provide not only insights into the subcellular distribution of Acheron and its potential binding partners but may also help elucidate its roles in programmed cell death, differentiation, and pathogenesis in mammalian models.