Although cancer is a diverse set of diseases, cancer cells share a number of adaptive hallmarks. Dysregulated pH is emerging as a hallmark of cancer because cancers show a 'reversed' pH gradient with a constitutively increased intracellular pH that is higher than the extracellular pH. This gradient enables cancer progression by promoting proliferation, the evasion of apoptosis, metabolic adaptation, migration and invasion. Several new advances, including an increased understanding of pH sensors, have provided insight into the molecular basis for pH-dependent cell behaviours that are relevant to cancer cell biology. We highlight the central role of pH sensors in cancer cell adaptations and suggest how dysregulated pH could be exploited to develop cancer-specific therapeutics.

Type I myosins are highly conserved actin-based molecular motors that localize to the actin-rich cortex and participate in motility functions such as endocytosis, polarized morphogenesis, and cell migration. The COOH-terminal tail of yeast myosin-I proteins, Myo3p and Myo5p, contains an Src homology domain 3 (SH3) followed by an acidic domain. The myosin-I SH3 domain interacted with both Bee1p and Vrp1p, yeast homologues of human WASP and WIP, adapter proteins that link actin assembly and signaling molecules. The myosin-I acidic domain interacted with Arp2/3 complex subunits, Arc40p and Arc19p, and showed both sequence similarity and genetic redundancy with the COOH-terminal acidic domain of Bee1p (Las17p), which controls Arp2/3-mediated actin nucleation. These findings suggest that myosin-I proteins may participate in a diverse set of motility functions through a role in actin assembly.

Post-translational modification of proteins is an evolutionarily conserved mechanism for regulating activity, binding affinities and stability. Compared with established post-translational modifications such as phosphorylation or uniquitination, post-translational modification by protons within physiological pH ranges is a less recognized mechanism for regulating protein function. By changing the charge of amino acid side chains, post-translational modification by protons can drive dynamical changes in protein conformation and function. Addition and removal of a proton is rapid and reversible and in contrast to most other post-translational modifications does not require an enzyme. Signaling specificity is achieved by only a minority of sites in proteins titrating within the physiological pH range. Here, we examine the structural mechanisms and functional consequences of proton post-translational modification of pH-sensing proteins regulating different cellular processes.

Phosphofructokinase-1 (PFK1), the “gatekeeper” of glycolysis, catalyses the committed step of the glycolytic pathway by converting fructose 6-phosphate (F6P) to fructose 1,6-bisphosphate. Allosteric activation and inhibition of PFK1 by over 10 metabolites and in response to hormonal signaling fine-tune glycolytic flux to meet energy requirements1. Mutations inhibiting PFK1 activity cause glycogen storage disease type VII, also known as Tarui disease2, and mice deficient in muscle PFK1 have decreased fat stores3. Additionally, PFK1 is suggested to have important roles in metabolic reprograming in cancer4,5. Despite its critical role in glucose flux, the biologically relevant crystal structure of the mammalian PFK1 tetramer has not been determined. We report here the first structures of the mammalian PFK1 tetramer, for the human platelet isoform (PFKP), in complex with ATP-Mg2+ and ADP at 3.1 and 3.4 Å, respectively. The structures reveal substantial conformational changes in the enzyme upon nucleotide hydrolysis as well as a unique tetramer interface. Mutations of residues in this interface can affect tetramer formation, enzyme catalysis and regulation, indicating the functional importance of the tetramer. With altered glycolytic flux being a hallmark of cancers6, these new structures allow a molecular understanding of the functional consequences of somatic PFK1 mutations identified in human cancers. We characterized three of these mutations and show they have distinct effects on allosteric regulation of PFKP activity and lactate production. The PFKP structural blueprint for somatic mutations as well as the catalytic site can guide therapeutic targeting of PFK1 activity to control dysregulated glycolysis in disease.

Phosphofructokinase-1 (PFK1) is an essential glycolysis enzyme as it catalyzes the step committing glucose to breakdown. Webb et al. show that the liver PFK1 isoform assembles into filaments in a tetramer- and substrate-dependent manner, providing insights into the spatial organization of isoform-specific glucose metabolism in cells.

The kinase Akt mediates signals from growth factor receptors for increased cell proliferation, survival, and migration, which contribute to the positive effects of Akt in cancer progression. Substrates are generally inhibited when phosphorylated by Akt; however, we show phosphorylation of the plasma membrane sodium-hydrogen exchanger NHE1 by Akt increases exchanger activity (H ؉ efflux). Our data fulfill criteria for NHE1 being a bona fide Akt substrate, including direct phosphorylation in vitro, using mass spectrometry and Akt phospho-substrate antibodies to identify Ser 648 as the Akt phosphorylation site and loss of increased exchanger phosphorylation and activity by insulin and platelet-derived growth factor in fibroblasts expressing a mutant NHE1-S648A. How Akt induces actin cytoskeleton remodeling to promote cell migration and tumor cell metastasis is unclear, but disassembly of actin stress fibers by platelet-derived growth factor and insulin and increased proliferation in growth medium are inhibited in fibroblasts expressing NHE1-S648A. We predict that other functions shared by Akt and NHE1, including cell growth and survival, might be regulated by increased H ؉ efflux.The serine/threonine kinase Akt/protein kinase B functions as a convergence site of cues that signal through the lipid kinase phosphoinositide 3-kinase (PI 3-kinase), 4 including activated integrin receptors, growth factor receptor tyrosine kinases, cytokine receptors, and G protein-coupled receptors. Akt confers signal relay from these upstream regulators to increase cell proliferation, cell survival, protein synthesis, and glycolytic flux (1). These actions contribute to the positive effects of Akt in cancer progression, and PI 3-kinase/Akt signaling is commonly aberrant in human tumors (2). Akt also regulates reorganization of the actin cytoskeleton, including disassembly of bundled actin stress fibers and formation of actin filament-rich membrane protrusions (3, 4), which likely enhance the positive effects of some Akt isoforms on cell migration (5) and tumor cell metastasis (6). However, compared with most cell processes regulated by Akt, our understanding of how Akt regulates cytoskeleton dynamics is least understood. Akt generally phosphorylates substrates at an RXRXX(S/T) recognition motif (7), and in most instances Akt phosphorylation inhibits activity or function of the substrate. Akt promotes cell proliferation by phosphorylating and inhibiting the negative cell cycle regulators p21Cip1/WAF1 (8) and p27 Kip1 (9 -11) and Wee1 kinase (12). It promotes cell survival by phosphorylating and inhibiting the function of proapoptosis regulators, including Bad (13,14) and forkhead transcription factors (15,16), and it increases protein synthesis by phosphorylating and inhibiting the negative regulators of mTOR, tubersclerosis sclerosis protein 2 (17) and proline-rich Akt substrate of 40 kDa (18). Additionally, Akt increases metabolism in part by phosphorylating and inhibiting glycogen synthase 3 (19). However, there are a few exceptions w...

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