Membrane Proteins

Troxacitabine (Troxatyl; BCH-4556; (-)-2'-deoxy-3'-oxacytidine), a deoxycytidine analogue with an unusual dioxolane structure and nonnatural L-configuration, has potent antitumor activity in animal models and is in clinical trials against human malignancies. The current work ...

Neuronal growth cones navigate over long distances along specific pathways to find their correct targets. The mechanisms and molecules that direct this pathfinding are the topics of this review. Growth cones appear to be guided by at least four different mechanisms: contact ...

Bacterial surface proteins are important molecules in the infectivity and survival of pathogens. Surface proteins on gram-positive bacteria have been shown to attach via a transpeptidase, termed sortase, that cleaves an LPXTG sequence found close to the C termini of nearly all surface proteins on these bacteria. We previously identified a unique enzyme (LPXTGase) from Streptococcus pyogenes that also cleaves the LPXTG motif with a catalytic activity higher than that of sortase, suggesting that it plays an important role in the attachment process. We have now purified and characterized an LPXTGase from Staphylococcus aureus and found that it has both similar and unique features compared to the S. pyogenes enzyme. The S. aureus enzyme is glycosylated and contains unusual amino acids, like its streptococcal counterpart. Like the streptococcal enzyme, staphylococcal LPXTGase has an overrepresentation of amino acids found in the peptidoglycan, i.e., glutamine/glutamic acid, glycine, alan...

Connexins (Cx) comprise a family of transmembrane proteins, which form intercellular channels between plasma membranes of two adjoining cells, commonly known as gap junctions. Recent reports revealed that Cx proteins interact with diverse cellular components to form a multiprotein complex, which has been termed "Nexus". Potential interaction partners include proteins such as cytoskeletal proteins, scaffolding proteins, protein kinases and phosphatases. These interactions allow correct subcellular localization of Cxs and functional regulation of gap junction-mediated intercellular communication. Evidence is accruing that Cxs might have channel-independent functions, which potentially include regulation of cell migration, cell polarization and growth control. In the current review, we summarize recent knowledge on Cx interactions with cytoskeletal proteins and highlight some aspects of their role in cellular motility.

Background: Folate deficiency is common in alcoholic patients, in part due to abnormal transport across membranes relevant to folate homeostasis. The reduced folate carrier (RFC) transports monoglutamyl folates across tissue membranes and could be affected by chronic exposure to ethanol. The micropig model is suitable to study the effect of alcoholism on RFC and folate transport across membranes.Methods: The membrane transport of [3H]-folic acid was measured by a vacuum filtration method in jejunal brush border (JBB), liver plasma membrane (LPM), and kidney brush border (KBB) membranes vesicles from micropigs fed control or 40% ethanol diets for 12 months. RFC transcripts were analyzed by reverse transcription polymerase chain reaction in jejunal mucosa, liver, and kidney from the same animals.Results: When we compared results from three relevant membranes in control animals, the transport of [3H]-folic acid was highest in LPM, 3-fold lower in KBB (p < 0.001), and 6-fold lower in JBB (p < 0.001). The concentration of RFC transcripts per total RNA was greatest in liver, followed by kidney and jejunum. The transport of [3H]-folic acid by JBB vesicles from chronic ethanol-fed animals exhibited 2-fold lower Km and Vmax (p < 0.05), whereas there was no ethanol effect on the Vmax of [3H]-folic acid transport by LPM or KBB. RFC transcript levels were 10-fold lower in jejunal mucosa from ethanol-fed animals than in control-fed animals (p < 0.005).Conclusions: Although our findings demonstrate different RFC transcript amounts and transport efficiencies among tissues, the present studies suggest that chronic ethanol exposure decreases the intestinal absorption of folic acid by altering the expression of RFC and consequently its transport kinetics in JBB. These findings provide a mechanism for the clinical finding of reduced folic acid absorption in chronic alcoholics.

Store-operated calcium entry (SOCE) is the predominant Ca2+ entry mechanism in nonexcitable cells and controls a variety of physiological and pathological processes. Although significant progress has been made in identifying the components required for SOCE, the molecular mechanisms underlying it are elusive. The present study provides evidence for a direct involvement of kinase suppressor of Ras 2 (KSR2) in SOCE. Using lymphocytes and fibroblasts from ksr2−/− mice and shKSR2-depleted cells, we find that KSR2 is critical for the elevation of cytosolic Ca2+ concentration. Specifically, our results show that although it is dispensable for Ca2+-store depletion, KSR2 is required for optimal calcium entry. We observe that KSR2 deficiency affects stromal interaction molecule 1 (STIM1)/ORAI1 puncta formation, which is correlated with cytoskeleton disorganization. Of interest, we find that KSR2-associated calcineurin is crucial for SOCE. Blocking calcineurin activity impairs STIM1/ORAI1 pun...

Filamentous, heterocyst-forming cyanobacteria exchange nutrients and regulators between cells for diazotrophic growth. Two alternative modes of exchange have been discussed involving transport either through the periplasm or through septal junctions linking adjacent cells. Septal junctions and channels in the septal peptidoglycan are likely filled with septal junction complexes. While possible proteinaceous factors involved in septal junction formation, SepJ (FraG), FraC, and FraD, have been identified, little is known about peptidoglycan channel formation and septal junction complex anchoring to the peptidoglycan. We describe a factor, SjcF1, involved in regulation of septal junction channel formation in the heterocyst-forming cyanobacterium Anabaena sp. strain PCC 7120. SjcF1 interacts with the peptidoglycan layer through two peptidoglycan-binding domains and is localized throughout the cell periphery but at higher levels in the intercellular septa. A strain with an insertion in s...