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Mannose‐specific lectins bind α‐2‐macroglobulin and an unknown protein from human plasma
Author(s) -
Van Leuven F.,
Torrekens S.,
Van Berghe H. Den,
Van Damme E.,
Peumans W.
Publication year - 1993
Publication title -
protein science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 3.353
H-Index - 175
eISSN - 1469-896X
pISSN - 0961-8368
DOI - 10.1002/pro.5560020214
Subject(s) - biochemistry , isoelectric point , lectin , mannose , biology , protein subunit , microbiology and biotechnology , chemistry , gene , enzyme
GNA, the mannose‐specific lectin from Galanthus nivalis was confirmed to bind α‐2‐macroglobulin (A2M) but another protein was copurified with A2M from total human plasma. A total of 23 other lectins with diverse specificities were tested for reaction with human A2M and with three other members of the A2M family. NPA, a mannose‐specific lectin isolated from Narcissus pseudonarcissus bulbs, and RSA, the Rhizoctonia solani agglutinin, were selected for further testing. For isolation of A2M, immobilized NPA was superior to GNA because its binding capacity was an order of magnitude higher. The specificity of these lectins must be very similar however, because the same unknown plasma protein was also bound by NPA. A2M and the unknown protein must share a unique mannose carbohydrate structure not present in any other human plasma protein. The copurified protein subunit size of 185 kDa is very similar to that of A2M, but the native molecular mass of 350 kDa indicated a noncovalent homodimer structure. Together with the acid isoelectric point this is not typical for any known plasma protein nor for any unidentified spot on the two‐dimensional map of human plasma proteins. No immunological reaction with available antisera was evident. A specific antiserum raised to the unknown protein demonstrated its presence in all human plasma samples examined. The N‐terminal residue was blocked, whereas internal protein sequences obtained after CNBr fragmentation and proteolysis were not homologous to any known protein sequence. These data demonstrate that this protein is unknown and not a proteinase inhibitor of the A2M family.