How presence of a signal peptide affects human galectins-1 and -4: Clues to explain common absence of a leader sequence among adhesion/growth-regulatory galectins.

BACKGROUND: Galectins are multifunctional effectors, which all share absence of a signal sequence. It is not clear why galectins belong to the small set of proteins, which avoid the classical export route. METHODS: Products of recombinant galectin expression in P. pastoris were analyzed by haemagglutination, gel filtration and electrophoresis and lectin blotting as well as mass spectrometry on the level of tryptic peptides and purified glycopeptides(s). Density gradient centrifugation and confocal laser scanning microscopy facilitated localization in transfected human and rat cells, proliferation assays determined activity as growth mediator. RESULTS: Directing galectin-1 to the classical secretory pathway in yeast produces N-glycosylated protein that is active. It cofractionates and -localizes with calnexin in human cells, only Gal-4 is secreted. Presence of N-glycan(s) reduces affinity of cell binding and growth regulation by Gal-1. CONCLUSIONS: Folding and activity of a galectin are maintained in signal-peptide-directed routing, N-glycosylation occurs. This pathway would deplete cytoplasm and nucleus of galectin, presence of N-glycans appears to interfere with lattice formation. GENERAL SIGNIFICANCE: Availability of glycosylated galectins facilitates functional assays to contribute to explain why galectins invariably avoid classical routing for export.

Galectins: their network and roles in immunity/tumor growth control.

One route of realizing the information of glycans involves endogenous receptors (lectins). Occurrence at branch ends renders galactosides particularly accessible. Thus, they are suited for such a recognition process. Fittingly, these epitopes serve as physiological ligands. The ga(lactoside-binding) lectins share the ß-sandwich fold with a sequence signature around a central tryptophan residue besides this specificity. Three modes of presentation of the carbohydrate recognition domain are known for galectins, and genome monitoring from fungi to mammals discloses that galectins form a network. The extent of its complexity varies considerably between organisms, for chicken reaching seven proteins, more for mammals. The current status of network analysis reveals overlapping and distinct expression profiles. Matching intra- and extracellular galectin presence, they have a broad range of functions at each site depending on their specific counterreceptor(s), with the possibility even for functional antagonism between family members. Orchestration of expression of galectin, the cognate glycan, its scaffold (protein or sphingolipid) and spatial aspects of glycoconjugate presentation has been detected to lead to growth regulation of immune and tumor cells. To delineate the factors that underlie the specificity of a galectin for its counterreceptor(s) in the cellular context and the details of structure-activity relationships by comparatively analyzing natural and rationally engineered proteins is the main challenge for ongoing research.