Proteins Connecting the Nuclear Pore Complex with the Nuclear Interior

Abstract

While much has been learned in recent years about the movement of soluble transport factors across the nuclear pore complex (NPC), comparatively little is known about intranuclear trafficking. We isolated the previously identified Saccharomyces protein Mlp1p (myosin-like protein) by an assay designed to find nuclear envelope (NE) associated proteins that are not nucleoporins. We localized both Mlp1p and a closely related protein that we termed Mlp2p to filamentous structures stretching from the nucleoplasmic face of the NE into the nucleoplasm, similar to the homologous vertebrate and Drosophila Tpr proteins. Mlp1p can be imported into the nucleus by virtue of a nuclear localization sequence (NLS) within its COOH-terminal domain. Overexpression experiments indicate that Mlp1p can form large structures within the nucleus which exclude chromatin but appear highly permeable to proteins. Remarkably, cells harboring a double deletion of MLP1 and MLP2 were viable, although they showed a slower net rate of active nuclear import and faster passive efflux of a reporter protein. Our data indicate that the Tpr homologues are not merely NPC-associated proteins but that they can be part of NPC-independent, peripheral intranuclear structures. In addition, we suggest that the Tpr filaments could provide chromatin-free conduits or tracks to guide the efficient translocation of macromolecules between the nucleoplasm and the NPC.

The SESA network links duplication of the yeast centrosome with the protein translation machinery

“…Ndc1 is an integral membrane protein that has overlapping functions with Pom152 and Pom34 in NPC biogenesis (Chial et al 1998;Tcheperegine et al 1999;Madrid et al 2006). The myosin like Mlp1 and Mlp2 are associated with the nuclear side of the NPC and have diverse roles in the retention of nonspliced mRNAs, nuclear transport, and SPB duplication (Strambio-de-Castillia et al 1999;Galy et al 2004;Niepel et al 2005). The mps2D suppression phenomenon was restricted to these NPC components.…”

Actively Transcribed GAL Genes Can Be Physically Linked to the Nuclear Pore by the SAGA Chromatin Modifying Complex

“…The evolutionary conservation of both the SAGA complex (human TBP-free TAF-containing complex (TFTC), p300-and CBP-associated factor (PCAF), and SPT3-TAF31-GCN5 acetyltransferase (STAGA) complexes (44 -47)) and the components of the NPC including the Mlp proteins (human translocated promoter region (Tpr) (15)) implies that locus recruitment to the NPC could in principle occur in higher eukaryotes by mechanisms similar to those in yeast. Furthermore, the Drosophila Mlp homolog, Mtor, is required for proper localization of the MSL histone acetyltransferase complex to the male X chromosome, where the MSL complex upregulates transcription of X-linked genes on the single copy of the male X chromosome at the nuclear periphery (12).…”

“…These components of the NPC include the Mlp proteins, Nic96, Nup1, Nup2, Nup60, and Nup116 (5,7,9). We focused on the two Mlp proteins as they are localized to the nuclear basket of the NPC (15), have roles in mRNA processing and export (16 -18), and have been implicated in the recruitment of multiple and diverse genes including the GAL genes, ␣ factor-responsive genes, and many other genes that are highly expressed under normal growth conditions (5,7). The two evolutionarily conserved Mlp proteins, Mlp1 and Mlp2, are large (218 and 195 kDa, respectively), coiled-coil domain proteins localized to the nuclear side of the NPC where they are hypothesized to form the intranuclear filaments of the inner basket of the NPC (15).…”

“…The two evolutionarily conserved Mlp proteins, Mlp1 and Mlp2, are large (218 and 195 kDa, respectively), coiled-coil domain proteins localized to the nuclear side of the NPC where they are hypothesized to form the intranuclear filaments of the inner basket of the NPC (15). The Mlp proteins, which are 66% similar, consist of extensive NH 2 -terminal coiled-coil domains and globular acidic carboxyl-terminal domains (15), the latter of which have been implicated in mRNA export and quality control via interactions with mRNA export factors (16 -19). Several observations have suggested that the Mlp proteins are linked to actively transcribed genes (5,7,12,19,20).…”

“…We focused on the two Mlp proteins as they are localized to the nuclear basket of the NPC (15), have roles in mRNA processing and export (16 -18), and have been implicated in the recruitment of multiple and diverse genes including the GAL genes, ␣ factor-responsive genes, and many other genes that are highly expressed under normal growth conditions (5,7). The two evolutionarily conserved Mlp proteins, Mlp1 and Mlp2, are large (218 and 195 kDa, respectively), coiled-coil domain proteins localized to the nuclear side of the NPC where they are hypothesized to form the intranuclear filaments of the inner basket of the NPC (15). The Mlp proteins, which are 66% similar, consist of extensive NH 2 -terminal coiled-coil domains and globular acidic carboxyl-terminal domains (15), the latter of which have been implicated in mRNA export and quality control via interactions with mRNA export factors (16 -19).…”