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

Abstract

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.

Keywords: Glycosylation; Golgi; Lectin; Secretion; Signal peptide; Signal sequence.