<p>The genes encoding the Ras family of small GTPases are mutated to yield constitutively active GTP-bound oncoproteins in one-third of all human cancers. In many other cancers lacking Ras mutations, Ras is activated by other means. One common example of such activation is found in breast cancer, in which epidermal growth factor receptor (EGFR) family receptor tyrosine kinases, including EGFR and HER2 (ErbB-2/Neu), are frequently amplified and overexpressed, which in turn activates Ras. In human cells, activation of the Ral guanine nucleotide exchange factor, or RalGEF, effector pathway is necessary for Ras-mediated tumorigenesis and metastasis. RalGEFs activate the two highly similar Ral GTPases, RalA and RalB. While RalA has been shown to be required for Ras-mediated tumorigenesis, RalB is important for tumor metastasis. Activated Ral GTPases bind to and activate a limited number of effector proteins, including RalBP1, Sec5, and Exo84, to affect numerous diverse activities of the cell. This dissertation research sought to determine which of these well-characterized Ral effector proteins were required for oncogenic mutant Ras-induceded tumorigenesis and metastasis of human cells, as well as to examine the role of RalA in breast cancer cells that can activate Ras through EGFR and HER2 overexpression. </p><p> RNA interference-mediated loss-of-function analysis demonstrated that Sec5 and Exo84 are required for oncogenic Ras-mediated tumorigenesis, and, at least in part, metastasis. Additionally, both gain-of-function and inhibition studies showed that RalA activation is induced by EGFR and HER2 in breast cancer cell lines stimulated with EGF. Furthermore, stable suppression of RalA expression inhibited tumorigenic growth of breast cancer cells, and RalA activation was shown to be higher in a majority of mammary adenocarcinomas versus matched patient normal mammary tissue. These studies provide new insights into the importance of RalA activation in breast cancer, as well as the molecules downstream of RalA and RalB that may be responsible for mediating their effects on tumorigenesis and metastasis.</p> / Dissertation

The Chromosomal Passenger Complex (CPC) is a major regulator of mitosis composed of the catalytic subunit Aurora B, the inner centromere protein INCENP, Survivin and Borealin/Dasra B. The CPC controls aspects of mitosis, ranging from chromosome and spindle structure to the correction of kinetochore-microtubule attachment errors, regulation of mitotic progression and completion of cytokinesis (Carmena et al., 2012). Knocking down any one CPC component induces delocalization of the others and disrupts mitotic progression (Adams et al., 2001 ; Carvalho et al., 2003; Lens et al., 2003 ; Gassmann et al., 2004; Vader et al., 2006). Telophase Disc (TD-60), also known as RCC2, is a putative Guanine Exchange Factor (GEF) that has been suggested to be involved in completion of cytokinesis through GTPase activation (Mollinari et al., 2003). However, its mechanism of action is still unclear. Interestingly, TD-60 has a typical Chromosomal Passenger Complex (CPC) localization (Andreassen et al., 1991) and its down-regulation alters CPC localisation during early mitosis. However, it is not a member of the CPC immunoprecipitated from mitotic cells (Gassmann et al., 2004). Here, I improved human TD-60 recombinant protein production by expressing a synthetic cDNA in the baculovirus expression system. This allowed me to characterize TD-60-associated GEF activity in vitro and study its possible influence on core CPC activity in vivo. I tested purified human TD-60 against a broad selection of GTPase targets, representing each GTPase family, in an established GEF assay. My data demonstrated that TD-60 has consistent high GEF activity in vitro towards the Ras-like protein A, RalA. To understand if TD-60 links RalA GTPase function to the CPC in vivo, I performed TD-60 and RalA RNAi experiments in HeLa cells. Interestingly, both TD-60 and RalA-depleted cells exhibit destabilized kinetochore fibers, a similar defective prometaphase-like bipolar spindle structure, and an abnormal centromeric accumulation of the CPC in early mitosis. In order to confirm that phenotypes seen after TD-60 depletion were due to lack of RalA activation in vivo, I generated a constitutively active RalA mutant that I transfected into TD-60- deficient cells. Strikingly, the RalA Q72L active mutant (mimicking the GTP-bound form) rescued the abnormal bipolar spindle structure, corrected the defective kinetochore-microtubules attachments, and rescued the atypical CPC distributions observed at centromeres after TD-60 depletion. These results suggest that TD-60-associated RalA GEF activity stabilizes kinetochore-microtubule attachments in early mitosis and that, TD-60 links RalA GTPase function to the CPC during mitosis.

<p>The Ras family of proteins, composed of H, N, and KRas, function as small GTPases that act as "molecular switches" relaying signals from the cell membrane to the rest of the cell in a highly regulated manner. However, Ras signaling is aberrantly activated in a majority of human cancers either through an activating mutation or by activation or overexpression of upstream or downstream elements in the Ras pathway, endowing cells with many tumorigenic phenotypes. Ras is known to promote tumorigenesis through activation of cell intrinsic signaling including the Raf, PI3K, and RalGEF pathways. In regards to the latter, RalGEFs activate two other small GTPases, RalA and RalB. The role of these two proteins in Ras-mediated cancer was poorly understood. I thus assessed the requirement of RalA and RalB in tumor metastasis discovering that both proteins promote this critical step in cancer.</p><p>Ras does not, however, function solely by intrinsic cell signaling. Indeed, it was recently shown that oncogenic Ras signaling induces secretion of cytokines, a category of small molecules involved in cell to cell communication and inflammatory response. Moreover, the release of these cytokines was shown to promote tumorigenesis in an extrinsic fashion by increasing tumor vasculature, or angiogenesis. I noted that one of these cytokines hCXCL-8 (IL-8) belonged to the ELR+ CXC family of cytokines, suggesting that the entire family of ELR+ CXC cytokines may promote Ras driven tumorigenesis. Indeed, I found that expression of oncogenic Ras led to secretion of all ELR+ CXC chemokines in oncogenic Ras driven tumor cell lines, a mouse tumor, a human tumor, and were sometimes elevated in the serum of pancreatic cancer patients, the cancer most associated with oncogenic Ras mutations. Moreover, knockdown of one of these chemokines, hCXCL-1, in pancreatic cancer cells or genetic ablation of the receptor for these cytokines in mice, reduced Ras driven tumorigenesis. Taken together, these results suggest that oncogenic Ras also promotes tumorigenesis through a cell extrinsic pathway by secretion of ELR+ CXC chemokines.</p> / Dissertation

No description available.

Our understanding of the folding of membrane proteins lags behind that of soluble proteins due to the challenges posed by the exposure of hydrophobic regions during in vitro chemical denaturation and refolding experiments. While different folding models are accepted for soluble proteins, only the two-stage model and the long-range interactions model have been proposed so far for helical membrane proteins. To address our knowledge gap on how different membrane proteins traverse their folding landscapes, Chapter 2 investigates the structural features of SDS-denatured states and the kinetics for reversible unfolding of sensory rhodopsin II (pSRII), a retinal-binding photophobic receptor from Natronomonas pharaonis. pSRII is difficult to denature, and only SDS can dislodge the retinal chromophore without rapid aggregation. Even in 30% SDS (0.998 $\mathit{\Chi}_{SDS}$), pSRII retains the equivalent of six out of seven transmembrane helices, while the retinal binding pocket is disrupted, with transmembrane residues becoming more solvent-exposed. Folding of pSRII from an SDS-denatured state harbouring a covalently-bound retinal chromophore shows deviations from an apparent two-state behaviour. SDS denaturation to form the sensory opsin apo-protein is reversible. This chapter establishes pSRII as a new model protein which is suitable for membrane protein folding studies and has a unique folding mechanism that differs from those of bacteriorhodopsin and bovine rhodopsin. In Chapter 3, SDS-denatured pSRII, acid-denatured pSRII and sensory opsin obtained by hydroxylamine-mediated bleaching of pSRII were characterised by solution state NMR. 1D $^1$H and $^{19}$F NMR were first used to characterise global changes in backbone amide protons and tryptophan side-chains. Residue-specific changes in backbone amide chemical shifts and peak intensities in 2D [$^1$H,$^{15}$N]-correlation spectra were analysed. While only small changes in the chemical environment of backbone amides were detected, changes in backbone amide dynamics were identified as an important feature of SDS- and acid-denatured pSRII and sensory opsin. $^{15}$N relaxation experiments were performed to study the backbone amide dynamics of SDS-denatured pSRII, reflecting motions on different timescales, including fast fluctuations of NH bond vectors on the ps-ns timescale and the lack of exchange contributions on the µs timescale. These studies shed insight on differences in the unfolding pathways under different denaturing conditions and the crucial role of the retinal chromophore in governing the structural integrity and dynamics of the pSRII helical bundle. Hydrogen bonds play fundamental roles in stabilising protein secondary and tertiary structure, and regulating protein function. Successful detection of hydrogen bonds in denatured states and during protein folding would contribute towards our understanding on the unfolding and folding pathways of the protein. Previous studies have demonstrated residue-specific detection of stable and transient hydrogen bonds in small globular proteins by measuring $^1{\it J}_{NH}$ scalar coupling constants using NMR. In Chapter 4, different methods for measuring $^1{\it J}_{NH}$ scalar coupling were explored using RalA, a small GTPase with a mixed alpha/beta fold, as proof-of-concept. Detection of hydrogen bonds was then attempted with OmpX, a beta-barrel membrane protein, both in its folded state in DPC micelles and in the urea-denatured state. While $^1{\it J}_{NH}$ measurement holds promise for studying hydrogen bond formation, further optimisation of NMR experiments and utilisation of perdeuterated samples are required to improve the precision of such measurements in large detergent-membrane protein complexes. Naturally occurring split inteins can mediate spontaneous trans-splicing both in vivo and in vitro. Previous studies have demonstrated successful assembly of proteorhodopsin from two separate fragments consisting of helices A-B and helices C-G via a splicing site in the BC loop. To complement the in vitro unfolding/folding studies, pSRII assembly in vivo was attempted by introducing a splicing site in the loop region of the beta-hairpin constituting the BC loop of pSRII. The expression conditions for the N- and C-terminal pSRII-intein segments were optimised, and the two segments co-expressed. However, the native chromophore was not observed. Further optimisation is required for successful in vivo trans-splicing of pSRII and application of this approach towards understanding the roles of helices and loops in the folding of pSRII.