Many genes are recruited to the nuclear periphery upon transcriptional activation in Saccharomyces cerevisiae. We have identified two Gene Recruitment Sequences (GRS I and II) from the promoter of the INO1 gene that target the gene to the nuclear periphery. These GRSs function as DNA zip codes; they are sufficient to target a nucleoplasmic locus to the nuclear periphery. Targeting requires components of the nuclear pore complex (NPC) and a GRS is sufficient to confer a physical interaction with the NPC. GRS I elements are enriched in promoters of genes that interact with the NPC and genes that are induced by protein folding stress. Full transcriptional activation of INO1 and another GRS-containing gene requires GRS-mediated targeting of the promoter to the nuclear periphery. Finally, GRS I also functions as a DNA zip code in Schizosaccharomyces pombe, suggesting that this mechanism of targeting to the nuclear periphery has been conserved over approximately one billion years of evolution.

The RAC guanine nucleotide binding proteins regulate multiple biological activities, including actin polymerization, activation of the Jun kinase (JNK) cascade, and cell proliferation. RAC effector loop mutants were identified that separate the ability of RAC to interact with different downstream effectors. One mutant of activated human RAC protein, RACV12H40 (with valine and histidine substituted at position 12 and 40, respectively), was defective in binding to PAK3, a Ste20-related p21-activated kinase (PAK), but bound to POR1, a RAC-binding protein. This mutant failed to stimulate PAK and JNK activity but still induced membrane ruffling and mediated transformation. A second mutant, RACV12L37 (with leucine substituted at position 37), which bound PAK but not POR1, induced JNK activation but was defective in inducing membrane ruffling and transformation. These results indicate that the effects of RAC on the JNK cascade and on actin polymerization and cell proliferation are mediated by distinct effector pathways that diverge at the level of RAC itself.

A role for POR1, a Rac1-interacting protein, in ARF6-mediated cytoskeletal rearrangements in amino acid sequence (Tsuchiya et al., 1991) on the gross morphology of the peripheral membrane remodels the actin cytoskeleton by inducing actin system (Peters et al., 1995). Expression of the wild-type polymerization at the cell periphery. This cytoskeletal and the GTPase-defective mutant, ARF6(Q67L), results rearrangement was inhibited by co-expression of in the elaboration of the plasma membrane characterized ARF6(Q67L) with deletion mutants of POR1, a Rac1-by the formation of extensive membrane vaginations, interacting protein involved in membrane ruffling, while expression of the ARF6(T27N) mutant defective in but not with the dominant-negative mutant of Rac1, GTP binding results in the massive accumulation of coated Rac1(S17N). A synergistic effect between POR1 and endosomes around the pericentriolar region of the cell. ARF6 for the induction of actin polymerization wasHere we report that the expression of the GTPase-defective detected. Furthermore, we observed that ARF6 intermutant of ARF6, ARF6(Q67L) in Chinese hamster ovary acts directly with POR1 and that this interaction was (CHO) cells induces a rearrangement of the actin cytoskel-GTP dependent. These findings indicate that ARF6eton with a redistribution of cortical actin to the cell and Rac1 function on distinct signaling pathways to periphery. The actin cytoskeleton has been implicated mediate cytoskeletal reorganization, and suggest a role in many cellular functions, including endocytosis, cell for POR1 as an important regulatory element in division, cell proliferation and cell motility. Therefore, orchestrating cytoskeletal rearrangements at the cell an understanding of how actin filament organization is periphery induced by ARF6 and Rac1.orchestrated is a central question in cell biology. Keywords: ARF6/cytoskeletal rearrangements/POR1/Rac A number of observations have implicated the Rho family of GTPases and growth factors in signal transduction pathways that regulate the actin cytoskeletal network

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