Occurrence of oligosialic acids on integrin α5 subunit and their involvement in cell adhesion to fibronectin

Integrin alpha (5)beta (1), a major fibronectin receptor, functions in a wide variety of biological phenomena. We have found that alpha2-8-linked oligosialic acids with 5:5 degree of polymerization (DP) less than or equal to 7 occur on integrin alpha (5) subunit of the human melanoma cell line G361. The integrin alpha (5) subunit immunoprecipitated with anti-integrin alpha (5) antibody reacted with the monoclonal antibody 12E3, which recognizes oligo/polysialic acid with DP greater than or equal to 5 but not with the polyclonal antibody H.46 recognizing oligo/polysialic acid with DP greater than or equal to 8. The occurrence of oligosialic acids was further demonstrated by fluorometric C-7/C-9 analysis on the immunopurified integrin alpha (5) subunit. Oligosialic acids were also found in the alpha (5) subunit of several other human cells such as foreskin fibroblast and chronic erythroleukemia K562 cells. These results suggest the ubiquitous modification with unique oligosialic acids occurs on the ar, subunit of integrin alpha (5)beta (1). The adhesion of human melanoma G361 cells to fibronectin was mainly mediated by integrin alpha (5)beta (1). Treatment of cells with sialidase from Arthrobacter ureafaciens cleaving alpha2-3-, alpha2-6-, and alpha2-8-linked sialic acids inhibited adhesion to fibronectin. On the other hand, N-acetyl-neuraminidase II, which cleaves alpha2-3 and alpha2-6 but not alpha2-8 linkages, showed no inhibitory activity. After the loss of oligosialic acids, integrin alpha (5)beta (1), failed to bind to fibronectin-conjugated Sepharose, indicating that the oligosialic acid on the a5 subunit of integrin alpha (5)beta (1) plays important roles in cell adhesion to fibronectin.