Tyrosinated, detyrosinated and acetylated tubulin isotypes in rat brain membranes. Their proportions in comparison with those in cytosol

Tubulin-Na+ , K + -ATPase interaction: Involvement in enzymatic regulation and cellular function

A new function for tubulin was described by our laboratory: acetylated tubulin forms a complex with Na⁺ ,K ⁺ -ATPase (NKA) and inhibits its activity. This process was shown to be a regulatory factor of physiological importance in cultured cells, human erythrocytes, and several rat tissues. Formation of the acetylated tubulin-NKA complex is reversible. We demonstrated that in cultured cells, high concentrations of glucose induce translocation of acetylated tubulin from cytoplasm to plasma membrane with a consequent inhibition of NKA activity. This effect is reversed by adding glutamate, which is coctransported to the cell with Na ⁺ . Another posttranslational modification of tubulin, detyrosinated tubulin, is also involved in the regulation of NKA activity: it enhances the NKA inhibition induced by acetylated tubulin. Manipulation of the content of these modifications of tubulin could work as a new strategy to maintain homeostasis of Na ⁺ and K ⁺ , and to regulate a variety of functions in which NKA is involved, such as osmotic fragility and deformability of human erythrocytes. The results summarized in this review show that the interaction between tubulin and NKA plays an important role in cellular physiology, both in the regulation of Na ⁺ /K ⁺ homeostasis and in the rheological properties of the cells, which is mechanically different from other roles reported up to now.

Mammalian plasma membrane proteins as potential biomarkers and drug targets

Defining the plasma membrane proteome is crucial to understand the role of plasma membrane in fundamental biological processes. Change in membrane proteins is one of the first events that take place under pathological conditions, making plasma membrane proteins a likely source of potential disease biomarkers with prognostic or diagnostic potential. Membrane proteins are also potential targets for monoclonal antibodies and other drugs that block receptors or inhibit enzymes essential to the disease progress. Despite several advanced methods recently developed for the analysis of hydrophobic proteins and proteins with posttranslational modifications, integral membrane proteins are still under-represented in plasma membrane proteome. Recent advances in proteomic investigation of plasma membrane proteins, defining their roles as diagnostic and prognostic disease biomarkers and as target molecules in disease treatment, are presented.

Plasma membrane tubulin

The association of tubulin with the plasma membrane comprises multiple levels of penetration into the bilayer: from integral membrane protein, to attachment via palmitoylation, to surface binding, and to microtubules attached by linker proteins to proteins in the membrane. Here we discuss the soundness and weaknesses of the chemical and biochemical evidence marshaled to support these associations, as well as the mechanisms by which tubulin or microtubules may regulate functions at the plasma membrane.

Brain plasma membrane Na+,K+-ATPase is inhibited by acetylated tubulin

Membranes from brain tissue contain tubulin that can be isolated as a hydrophobic compound by partitioning into Triton X-114. The hydrophobic behavior of this tubulin is due to the formation of a complex with the alpha-subunit of Na+,K+-ATPase. In the present work we show that the interaction of tubulin with Na+K+-ATPase inhibits the enzyme activity. We found that the magnitude of the inhibition is correlated with: (1) concentration of the acetylated tubulin isoform present in the tubulin preparation used, and (2) amount of acetylated tubulin isoform isolated as a hydrophobic compound. In addition, some compounds involved in the catalytic action of Na+K+-ATPase were assayed to determine their effects on the inhibitory capability of tubulin on this enzyme. The inhibitory effect of tubulin was only slightly decreased by ATP at relatively low nucleotide concentration (0.06 mM). NaCl (1-160 mM) and KCl (0.2-10 mM) showed no effect whereas inorganic phosphate abolished the inhibitory effect of tubulin in a concentration-dependent manner.

Ultrastructural localization of nitrotyrosine within the caudate-putamen nucleus and the globus pallidus of normal rat brain

Nitration of protein tyrosine residues by nitric oxide (NO)-derived reactive species results in the production of stable nitrotyrosine (NT) moieties that are immunochemically detectable in many regions of normal brain and enriched in those areas containing constitutive nitric oxide synthase (cNOS). These include the caudate-putamen nucleus (CPN) and the globus pallidus, which receives major inhibitory input from the CPN. To determine the functional sites for NT production in these critical motor nuclei, we examined the electron microscopic immunocytochemical localization of NT and cNOS in rat brain. In the CPN, NT was localized to the somata and dendrites of cNOS-containing interneurons and spiny neurons, some of which received input from cNOS-labeled terminals. The NT immunoreactivity was most prevalent on outer mitochondrial membranes and nearby segments of the plasma membranes in dendrites and within asymmetric synapses on dendritic spines. In the CPN and globus pallidus, there was also a prominent labeling of NT in astrocytic processes, small axons, and tubulovesicles and/or synaptic vesicles in axon terminals. These terminals formed mainly asymmetric synapses in the CPN and inhibitory-type synapses in the globus pallidus where they often apposed cNOS-containing terminals that also formed asymmetric, excitatory-type synapses. Our results suggest that NT is generated by mechanisms requiring the dual actions of excitatory transmitters and NO derived either from interneurons in the CPN or from excitatory afferents in the globus pallidus. The findings also implicate NT in the physiological actions of NO within the striatal circuitry and, particularly, in striatopallidal neurons severely affected in Huntington's disease.

Na+,K+-ATPase was found to be the membrane component responsible for the hydrophobic behavior of the brain membrane tubulin

We have previously described that the tubulin isolated from brain membranes as a hydrophobic compound by partitioning into Triton X-114 is a peripheral membrane protein [corrected]. The hydrophobic behavior of this tubulin is due to its interaction with membrane protein(s) and the interaction occurs principally with the acetylated tubulin isotype. In the present work we identified the membrane protein that interacts with tubulin as the Na+,K+-ATPase alpha subunit by amino acid sequencing. Using purified brain Na+,K+-ATPase we were able to isolate part of the total hydrophilic tubulin as a hydrophobic compound which contains a high proportion of the acetylated tubulin isotype.

Interaction of cytoskeletal proteins with membrane lipids

Rapid and significant progress has been made in understanding lipid/protein interactions involving cytoskeletal components and the plasma membrane. Covalent and noncovalent lipid modifications of cytoskeletal proteins mediate their interaction with lipid bilayers. The application of biophysical techniques such as differential scanning colorimetry, neutron reflection, electron spin resonance, CD spectroscopy, nuclear magnetic resonance, and hydrophobic photolabeling, allow various folding stages of proteins during electrostatic adsorption and hydrophobic insertion into lipid bilayers to be analyzed. Reconstitution of proteins into planar lipid films and liposomes help to understand the architecture of biological interfaces. During signaling events at plasma membrane interfaces, lipids are important for the regulation of catalytic protein functions. Protein/lipid interactions occur selectively and with a high degree of specificity and thus have to be considered as physiologically relevant processes with gaining impact on cell functions.

The relationship of hydrophobic tubulin with membranes in neural tissue

Brain membrane preparations contain tubulin that can be extracted with Triton X-114. After the extract is allowed to partition, 8% of the total brain tubulin is isolated as a hydrophobic compound in the detergent-rich phase. Cytosolic tubulin does not show this hydrophobic behaviour since it is recovered in the aqueous phase. Membrane tubulin can be released by 0.1 M Na2 CO3 treatment at pH > or = 11.5 in such a way that the hydrophobic tubulin is converted into the hydrophilic form. These results suggest that tubulin exists associated with some membrane component that confers the hydrophobic behaviour to tubulin. If the tissue is homogenized in microtubule-stabilizing buffer containing Triton X-100, the hydrophobic tubulin is isolated from the microtubule fraction. This result indicates that the hydrophobic tubulin isolated from membrane preparations belongs to microtubules that in vivo are associated to membranes. Therefore, hydrophobic tubulin (tubulin-membrane component complex) can be obtained from membranes or from microtubules depending on the conditions of brain homogenization.