The NH2 terminus of galectin-3 governs cellular compartmentalization and functions in cancer cells.
Galectin-3 is a member of the beta-galactoside-binding protein family shown to be involved in tumor progression and metastasis. It has a unique primary structure consisting of three domains: a 12-amino acid leader sequence containing a casein kinase I serine phosphorylation site, which is preceded by a collagenase-sensitive Pro-Gly-rich motif, and a COOH-terminal half encompassing the carbohydrate-binding site. To study the functional role of the unusual leader sequence of galectin-3, a mutant cDNA that causes an 11-amino acid deletion in the NH2-terminal region was generated and expressed in galectin-3-null BT-549 human breast carcinoma cells. Deletion of the NH2 terminus resulted in abolition of the secretion of truncated galectin-3, loss of nuclear localization, and reduced carbohydrate-mediated functions compared with the wild-type protein. When green fluorescent protein was fused to the galectin-3 leader sequence and transiently transfected into BT-549 cells, the uniform cellular distribution of native green fluorescent protein was changed mainly to a nuclear pattern. To further investigate whether the functional changes observed in a galectin-3 with the 11 NH2-terminal amino acids deleted were due to loss of phosphorylation at Ser6, two point mutations were created at this serine: Ser6-->Ala and Ser6-->Glu. No obvious difference was observed in cellular localization between wild-type and Ser6-mutated transfectants. These results suggest a structural role for the NH2 terminus leader motif of galectin-3 in determining its cellular targeting and biological functions independent of phosphorylation.
Gong
H C
HC
Metastasis Research Program, Karmanos Cancer Institute, Detroit, Michigan 48201, USA.
Honjo
Y
Y
Nangia-Makker
P
P
Hogan
V
V
Mazurak
N
N
Bresalier
R S
RS
Raz
A
A
D000943:Antigens, Differentiation / Q000096:biosynthesis / Q000235:genetics / Q000502:physiology*; D001665:Binding Sites; D001692:Biological Transport; D047388:Casein Kinases; D002451:Cell Compartmentation; D002455:Cell Division / Q000502:physiology; D002471:Cell Transformation, Neoplastic; D018076:DNA, Complementary; D037502:Galectin 3; D017353:Gene Deletion; D006384:Hemagglutination; D006801:Humans; D016297:Mutagenesis, Site-Directed; D009369:Neoplasms / Q000378:metabolism; D010446:Peptide Fragments / Q000502:physiology; D010766:Phosphorylation; D011494:Protein Kinases / Q000378:metabolism; D012694:Serine / Q000378:metabolism; D014162:Transfection; D014407:Tumor Cells, Cultured
D000943:Antigens, Differentiation; D018076:DNA, Complementary; D037502:Galectin 3; D010446:Peptide Fragments; D012694:Serine; D011494:Protein Kinases; D047388:Casein Kinases