Control of osteopontin signaling and function by post-translational phosphorylation and protein folding

Osteopontin (OPN) plays roles in a variety of cellular processes from bone resorption and extracellular matrix (ECM) remodeling to immune cell activation and inhibition of apoptosis. Because it binds receptors (integrins, CD44 variants) typically engaged by ECM molecules, OPN acts as a "soluble" ECM molecule. A persistent theme throughout the characterization of how OPN functions has been the importance of phosphorylation. The source of the OPN used in specific experiments and the location of modified sites is an increasingly important consideration for OPN research. We review briefly some of the ways OPN impacts on the biology of mammalian systems with an emphasis on the importance of serine phosphorylation in modulating its signaling ability. We describe experiments that support the hypothesis that differences in the post-translational phosphorylation of OPN expressed by different cell types regulate how it impacts on target cells. Analyses of OPN's potential secondary structure reveal a possible beta-sheet conformation that offers an interpretation of certain experimental observations, specifically the effect of thrombin cleavage; it is consistent with an interaction between the C-terminal region of the protein and the central integrin-binding RGD sequence.     

ASSOCIATION OF A FREE-LIVING EUGLENA WITH A COLONIAL ROTIFER

Large epiphytic and epilithic diatom species hosting other diatoms were observed in several fresh- and brackish water sites in Southern California. The most commonly encountered hosts were species forming long filaments attached to rocks or macroalgae, Hydrosera whampoensis (Schwarz) Hendey, Melosira varians Agardh, Pleurosira laevis (Ehrenberg) Compere and Terpsinoe musica Ehrenberg. These large diatoms often had smaller diatoms attached, usually to the girdle bands and occasionally to the mucilage pads connecting the cells. For example, cells of T. musica were observed supporting growth of a diverse diatom assemblage composed of species of the genera Achnanthes , Achnanthidium , Amphora , Cocconeis and Tabularia; Synedra sp. was attached to M. varians and B. paxillifer; and Cocconeis placentula was seen on H. whampoensis. Thus, large epiphytic and epilithic diatoms seem to provide suitable sites for attachment of small epiphytic diatom species, and it appears that this phenomenon is more common than previously thought.