Membrane traffic is tightly regulated and the Rab protein family of small GTPases plays a central role in this regulation. One member of this family is the Saccharomyces cerevisae protein Ypt6. To search for new genes interacting with Ypt6-related pathways, we performed a genetic screen for high copy suppressors of ypt6 Delta temperature sensitivity at 35 degrees C. Among the suppressors, MYR1 was also able to suppress the temperature sensitive mutant lacking Ric1, a subunit of the Ypt6 guanine exchanging factor complex Ric1/Rgp1. Myr1 is characterized by a coiled coil region and a GYF domain, a protein module binding proline-rich sequences. Myr1 is able to bind membranes but is also associated with larger structures insoluble in Triton X-100. By immunofluorescence, Myr1 shows a network-like pattern as well as small foci. Overexpression of Myr1 influences nuclear envelope morphology and high levels are lethal. This lethality is rescued when the N-terminal region, containing the GYF domain, is deleted. The transcription profile of a myr1 Delta strain shows effects on genes involved in nuclear migration, Ras signalling and transcription. Taken together, these results suggest that Myr1 is a novel factor linked to the secretory pathway and important cellular regulatory mechanisms.

Arginine methylation is a post-translational modification of proteins implicated in RNA processing, protein compartmentalization, signal transduction, transcriptional regulation and DNA repair. In a screen for proteins associated with the nuclear envelope in the yeast Saccharomyces cerevisiae, we have identified the arginine methyltransferase Rmt2, previously shown to methylate the ribosomal protein L12. By indirect immunofluorescence and subcellular fractionations we demonstrate here that Rmt2 has nuclear and cytoplasmic localizations. Biochemical analysis of a fraction enriched in nuclei reveals that nuclear Rmt2 is resistant to extractions with salt and detergent, indicating an association with structural components. This was supported by affinity purification experiments with TAP-tagged Rmt2. Rmt2 was found to co-purify with the nucleoporins Nup49, Nup57 and Nup100, revealing a novel link between arginine methyltransferases and the nuclear pore complex. In addition, a genome-wide transcription study of the rmt2 Delta mutant shows significant downregulation of the transcription of MYO1, encoding the Type II myosin heavy chain required for cytokinesis and cell separation.