Small GTP-binding proteins (G proteins) exist in eukaryotes from yeast to human and constitute a superfamily consisting of more than 100 members. This superfamily is structurally classified into at least five families: the Ras, Rho, Rab, Sar1/Arf, and Ran families. They regulate a wide variety of cell functions as biological timers (biotimers) that initiate and terminate specific cell functions and determine the periods of time for the continuation of the specific cell functions. They furthermore play key roles in not only temporal but also spatial determination of specific cell functions. The Ras family regulates gene expression, the Rho family regulates cytoskeletal reorganization and gene expression, the Rab and Sar1/Arf families regulate vesicle trafficking, and the Ran family regulates nucleocytoplasmic transport and microtubule organization. Many upstream regulators and downstream effectors of small G proteins have been isolated, and their modes of activation and action have gradually been elucidated. Cascades and cross-talks of small G proteins have also been clarified. In this review, functions of small G proteins and their modes of activation and action are described.

Small GTP-Binding Proteins

▪ Abstract The compartmentation of eukaryotic cells requires all nuclear proteins to be imported from the cytoplasm, whereas, for example, transfer RNAs, messenger RNAs, and ribosomes are made in the nucleus and need to be exported to the cytoplasm. Nuclear import and export proceed through nuclear pore complexes and can occur along a great number of distinct pathways, many of which are mediated by importin β-related nuclear transport receptors. These receptors shuttle between nucleus and cytoplasm, and they bind transport substrates either directly or via adapter molecules. They all cooperate with the RanGTPase system to regulate the interactions with their cargoes. Another focus of our review is nuclear export of messenger RNA, which apparently largely relies on export mediators distinct from importin β-related factors. We discuss mechanistic aspects and the energetics of transport receptor function and describe a number of pathways in detail.

Transport between the cell nucleus and the cytoplasm.

▪ Abstract Small ubiquitin-related modifier (SUMO) family proteins function by becoming covalently attached to other proteins as post-translational modifications. SUMO modifies many proteins that participate in diverse cellular processes, including transcriptional regulation, nuclear transport, maintenance of genome integrity, and signal transduction. Reversible attachment of SUMO is controlled by an enzyme pathway that is analogous to the ubiquitin pathway. The functional consequences of SUMO attachment vary greatly from substrate to substrate, and in many cases are not understood at the molecular level. Frequently SUMO alters interactions of substrates with other proteins or with DNA, but SUMO can also act by blocking ubiquitin attachment sites. An unusual feature of SUMO modification is that, for most substrates, only a small fraction of the substrate is sumoylated at any given time. This review discusses our current understanding of how SUMO conjugation is controlled, as well as the roles of SUMO in a...

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Protein Modification by SUMO

An understanding of how the nuclear pore complex (NPC) mediates nucleocytoplasmic exchange requires a comprehensive inventory of the molecular components of the NPC and a knowledge of how each component contributes to the overall structure of this large molecular translocation machine. Therefore, we have taken a comprehensive approach to classify all components of the yeast NPC (nucleoporins). This involved identifying all the proteins present in a highly enriched NPC fraction, determining which of these proteins were nucleoporins, and localizing each nucleoporin within the NPC. Using these data, we present a map of the molecular architecture of the yeast NPC and provide evidence for a Brownian affinity gating mechanism for nucleocytoplasmic transport.

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The Yeast Nuclear Pore Complex: Composition, Architecture, and Transport Mechanism

TORC1 (target of rapamycin complex 1) has a crucial role in the regulation of cell growth and size. A wide range of signals, including amino acids, is known to activate TORC1. Here, we report the identification of Rag GTPases as activators of TORC1 in response to amino acid signals. Knockdown of Rag gene expression suppressed the stimulatory effect of amino acids on TORC1 in Drosophila melanogaster S2 cells. Expression of constitutively active (GTP-bound) Rag in mammalian cells activated TORC1 in the absence of amino acids, whereas expression of dominant-negative Rag blocked the stimulatory effects of amino acids on TORC1. Genetic studies in Drosophila also show that Rag GTPases regulate cell growth, autophagy and animal viability during starvation. Our studies establish a function of Rag GTPases in TORC1 activation in response to amino acid signals.

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Regulation of TORC1 by Rag GTPases in nutrient response.

RAN/TC4 is a small nuclear G protein1 that forms a complex with the chromatin-bound guanine nucleotide release factor RCC1 (ref. 2). Loss of RCC1 causes defects in cell cycle progression3,4, RNA export5-7 and nuclear protein import8. Some of these can be suppressed by overexpression of Ran/TC4 (ref. 1), suggesting that Ran/TC4 functions downstream of RCC1. We have searched for proteins that bind Ran/TC4 by using a two-hybrid screen, and here we report the identification of RanBP2, a novel protein of 3,224 residues. This giant protein comprises an amino-terminal 700-residue leucine-rich region, four RanBPl-homologous (refs 9, 10) domains, eight zinc-finger motifs similar to those of NUP153 (refs 11, 12), and a carboxy terminus with high homology to cyclophilin13. The molecule contains the XFXFG pentapeptide motif characteristic of nuclear pore complex (NPC) proteins14, and immunolocalization suggests that RanBP2 is a constituent of the NPC. The fact that NLS-mediated nuclear import can be inhibited by an antibody directed against RanBP2 supports a functional role in protein import through the NPC.

A giant nucleopore protein that binds Ran/TC4.

Using the two-hybrid method, we isolated a Saccharomyces cerevisiae cDNA encoding a protein homologous to Schizosaccharomyces pombe protein Dis3sp, using as bait, human GTPase Ran. The DIS3 gene is essential for viability and complements S.pombe mutant dis3-54 which is defective in mitosis. Although Dis3sc has no homology to RanBP1, it bound directly to Ran and the S.cerevisiae Ran homologue Cnr1, but not to the S.cerevisiae RCC1 homologue Srm1. Upon binding to Ran with a 1:1 molar ratio, Dis3sc enhanced a nucleotide-releasing activity of RCC1 on Ran. In the presence of Dis3sc, the K(m) of RCC1 on Ran decreased by half, while the kcat was unchanged. In vivo, Dis3sp was present as oligomers of M(r) 670-200 kDa as previously reported, and the 200 kDa oligomer of Dis3sp was found to include Spi1 and Pim1, the S.pombe homologues of Ran and RCC1, respectively. Although the biological function of the heterotrimeric oligomer consisting of Dis3, Spi1 and Pim1 is unknown, our results indicate that Dis3 is a component of the RCC1-Ran pathway.

https://www.embopress.org/doi/pdf/10.1002/j.1460-2075.1996.tb00944.x

Dis3, implicated in mitotic control, binds directly to Ran and enhances the GEF activity of RCC1

Cerebrovascular reactivity to changes in the partial pressure of arterial carbon dioxide (P) via limiting changes in brain [H+] modulates ventilatory control. It remains unclear, however, how exercise-induced alterations in respiratory chemoreflex might influence cerebral blood flow (CBF), in particular the cerebrovascular reactivity to CO2. The respiratory chemoreflex system controlling ventilation consists of two subsystems: the central controller (controlling element), and peripheral plant (controlled element). In order to examine the effect of exercise-induced alterations in ventilatory chemoreflex on cerebrovascular CO2 reactivity, these two subsystems of the respiratory chemoreflex system and cerebral CO2 reactivity were evaluated (n= 7) by the administration of CO2 as well as by voluntary hypo- and hyperventilation at rest and during steady-state exercise. During exercise, in the central controller, the regression line for the P–minute ventilation relation shifted to higher and P with no change in gain (P= 0.84). The functional curve of the peripheral plant also reset rightward and upward during exercise. However, from rest to exercise, gain of the peripheral plant decreased, especially during the hypercapnic condition (−4.1 ± 0.8 to −2.0 ± 0.2 mmHg l−1 min−1, P= 0.01). Therefore, under hypercapnia, total respiratory loop gain was markedly reduced during exercise (−8.0 ± 2.3 to −3.5 ± 1.0 U, P= 0.02). In contrast, cerebrovascular CO2 reactivity at each condition, especially to hypercapnia, was increased during exercise (2.4 ± 0.2 to 2.8 ± 0.2% mmHg−1, P= 0.03). These findings indicate that, despite an attenuated chemoreflex system controlling ventilation, elevations in cerebrovascular reactivity might help maintain CO2 homeostasis in the brain during exercise.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.2008.157073

Interaction between the ventilatory and cerebrovascular responses to hypo‐ and hypercapnia at rest and during exercise

RCC1 is a chromosomal protein that functions as a GEF of the nuclear G protein Ran, which GTPase activity is enhanced by RNA1 located in the cytoplasm. RCC1 has no preference for GTP or GDP-bound Ran, so that GTP-Ran formation in vivo is regulated by relative concentrations of GTP/GDP and regulatory proteins interacting with RCC1, Ran, and RNA1. Proteins possessing the special Ran-binding motif have been found to be conserved in species ranging from yeasts to mammalians. The finding of RanBP2/NUP358 clearly indicates the involvement of the Ran pathway in the nuclear pore transport function, in agreement with the finding that both rcc1- and rna1- show defects in this process. However, loss of RCC1 induces premature initiation of mitosis, resulting in G1 arrest with the micronuclei possessing mitotic condensed chromosomes. How both the cell cycle and nucleocytoplasmic transport are regulated by the RCC1-Ran pathway is a major question.

RCC1 in the Ran Pathway

We examined whether lactic acidemia-induced hyperemia at the onset of high-intensity leg exercise contributed to the speeding of pulmonary O2 uptake (V˙o 2) after prior heavy exercise of the same m...

https://www.physiology.org/doi/pdf/10.1152/japplphysiol.00207.2001

Naoyuki Hayashi

Papers

A giant nucleopore protein that binds Ran/TC4.

Dis3, implicated in mitotic control, binds directly to Ran and enhances the GEF activity of RCC1

Interaction between the ventilatory and cerebrovascular responses to hypo‐ and hypercapnia at rest and during exercise

RCC1 in the Ran Pathway

V˙o 2 kinetics in heavy exercise is not altered by prior exercise with a different muscle group