| The nuclear envelope (NE) is the distinctive attribute of eukaryotic cells. It separates chromatin from the cytoplasm, provides an anchoring surface for chromatin and is involved in organizing nuclear architecture. One of the key functions of the NE during interphase is regulation of nucleocytoplasmic transport. It also plays a pivotal role in the complex dissociation and re-association of the nucleus during open mitosis. While the NE has extensively been studied in animals, the function as well as protein composition of the NE in plants is barely understood. LeMAF1 is a plant-specific, NE-associated protein first identified in tomato. It is the founding member of a group of WPP-domain containing NE-associated proteins that includes RanGAP and the Arabidopsis WPP family, comprised of WPP1, WPP2 and WPP3. Here, the localization and function of WPP-domain proteins were investigated. WPP1 and WPP2 are targeted to the NE in a developmentally regulated fashion. Also, LeMAF1 is associated with the outer NE and the nuclear pores in interphase cells, and WPP1 is associated with the immature cell plate during cytokinesis. Besides its NE-association, LeMAF1 localizes in cytoplasmic speckles that are components of the Golgi apparatus. Further, the binding of WPP-domain proteins to the Golgi-associated, coiled-coil protein, LeWAP, was investigated. The coiled-coil domain of LeWAP is the primary interaction domain with WPP-domain proteins. RNAi lines with undetectable expression of all three Arabidopsis WPP genes at the RNA as well as protein level have shorter primary roots and a reduced number of lateral roots. This is in turn caused by reduced numbers of dividing cells. Together, these data demonstrate the first example of regulated NE targeting in plants and identify a class of plant-specific NE and Golgi-associated proteins involved in mitotic activity. The molecular functions of WPP-domain proteins and whether those functions are interconnected at the NE and Golgi yet remains to be determined.; Lastly, the cloning and characterization of a transmembrane domain-containing, coiled-coil protein, TMD1, is described. The asymmetric localization of TMD1 at the anticlinal surface of the plasma membrane is actin dependent and mediated by Golgi vesicles. Arabidopsis TMD1 knockout mutant plants are highly branched and possess shorter siliques, hypocotyls and inflorescences. The latter two are caused by reduced cell elongation. Taken together, a role for TMD1 in auxin transport is postulated. |
