ABSTRACT
In eukaryotes, mRNAs are transcribed in the nucleus and exported to the cytoplasm for translation to occur. Messenger RNAs complexed with proteins referred to as ribonucleoparticles are recognized for nuclear export in part by association with Mex67, a key Saccharomyces cerevisiae mRNA export factor and homolog of human TAP/NXF1. Mex67, along with its cofactor Mtr2, is thought to promote ribonucleoparticle translocation by interacting directly with components of the nuclear pore complex (NPC). Herein, we show that the nuclear pore-associated protein Sac3 functions in mRNA export. Using a mutant allele of MTR2 as a starting point, we have identified a mutation in SAC3 in a screen for synthetic lethal interactors. Loss of function of SAC3 causes a strong nuclear accumulation of mRNA and synthetic lethality with a number of mRNA export mutants. Furthermore, Sac3 can be coimmunoprecipitated with Mex67, Mtr2, and other factors involved in mRNA export. Immunoelectron microscopy analysis shows that Sac3 localizes exclusively to cytoplasmic fibrils of the NPC. Finally, Mex67 accumulates at the nuclear rim when SAC3 is mutated, suggesting that Sac3 functions in Mex67 translocation through the NPC.
Subject(s)
Cytoskeleton/metabolism , Nuclear Pore/metabolism , Nuclear Proteins/metabolism , RNA, Messenger/metabolism , Saccharomyces cerevisiae Proteins/metabolism , Biological Transport/physiology , Humans , Nuclear Proteins/genetics , Nucleocytoplasmic Transport Proteins/genetics , Nucleocytoplasmic Transport Proteins/metabolism , Porins , RNA-Binding Proteins/genetics , RNA-Binding Proteins/metabolism , Recombinant Fusion Proteins/metabolism , Saccharomyces cerevisiae/metabolism , Saccharomyces cerevisiae/ultrastructure , Saccharomyces cerevisiae Proteins/geneticsABSTRACT
Two models have been put forward to explain the growth of new Golgi during the cell cycle. The first suggests that a new Golgi grows out of the endoplasmic reticulum by de novo synthesis. The second suggests that a pre-existing Golgi is needed for the growth of a new one, that is, the Golgi is an autonomously replicating organelle. To resolve this issue, we have exploited the simplicity of the apicomplexan parasite Toxoplasma gondii, which has only a single Golgi stack. Here we show, by using video fluorescence microscopy and three-dimensional reconstructions of serial thin sections, that the Golgi grows by a process of lateral extension followed by medial fission. Further fission leads to the inheritance by each daughter of a pair of Golgi structures, which then coalesce to re-form a single Golgi. Our results indicate that new Golgi grow by autonomous duplication and raise the possibility that the Golgi is a paired structure that is analogous to centrioles.
