Sitosterol and glucosylceramide cooperative transversal and lateral uneven distribution in plant membranes.

The properties of biomembranes depend on the presence, local structure and relative distribution assumed by the thousands of components it is made of. As for animal cells, plant membranes have been demonstrated to be organized in subdomains with different persistence lengths and times. In plant cells, sitosterol has been demonstrated to confer to phospholipid membranes a more ordered structure while among lipids, glycosphingolipids are claimed to form rafts where they tightly pack with sterols. Glucosylceramides are glycosphingolipids involved in plant signalling and are essential for viability of cells and whole plant. The glucosylceramide-sitosterol structural coupling within PLPC membranes is here investigated by Langmuir films, in silico simulations and neutron reflectometry, unveiling that a strong direct interaction between the two molecules exists and governs their lateral and transversal distribution within membrane leaflets. The understanding of the driving forces governing specific molecules clustering and segregation in subdomains, such as glucosylceramide and sitosterol, have an impact on the mechanical properties of biomembranes and could reflect in the other membrane molecules partitioning and activity.

N-linked sialyated sugar receptors support haematopoietic cell-osteoblast adhesions.

Haematopoietic progenitor cells proliferate and develop predominantly when they adhere to bone marrow stromal cells such as osteoblasts. Therefore, changes in adhesion may be a common mechanism by which stem cells survive, mature and properly traffic between the bone marrow and the circulation. To characterize these adhesion molecules, we reduced the bone marrow cavity to a simple adhesion assay between KG1a (a CD34+ haematopoietic cell line) and osteosarcoma monolayers (MG-63 or SaOS-2). The data demonstrated that adhesion was mediated by cell-to-cell rather than cell-to-matrix contact, was sensitive to trypsin, calcium chelators and glycosylation inhibitors. Selective pretreatment attributed the constitutive binding to N-linked glycans on KG1a. When carboprocessing was inhibited later at the high mannose intermediate (via deoxymannojirimycin), adhesion was retained. Surprisingly, binding of KG1a to SaOS-2 increased past constitutive levels as doses of tunicamycin or deoxymannojirimycin dropped. Selective pretreatment suggested that this 'inducible' binding resides with molecule(s) on SaOS-2. If the terminal sialic acid was digested (via neuraminidase), this induced response was duplicated. These data, verified in primary cells, suggest that the initial tethering between blood and bone cells in this model is probably due to heavily glycosylated, rapidly processed protein(s) on both cell types.