The role of eucaryotic cell surface glycoconjugates in DNA replication.

Multiple forms of ricin have been isolated from castor bean seeds. Two forms, ricin-1 and ricin-2, differ in their isoelectric pI values and toxicity towards IMR- 32 cells. Inhibition of IMR-32 DNA polymerase α2 is more pronounced with ricin-1 (65%) than with ricin-2 (10%). Ricin Β chain (pI = 5.2) isolated from ricin-1 binds to IMR-32 cell surfaces as well as inhibits DNA polymerase α2 activity when studied in vitro. The presence of galβ-linked glycoconjugates near the active site of IMR-32 DNA polymerase α2 has been proposed. Replication modulators which bind to the glycose portion of the enzymes involved in the replication system may need a mandatory binding to cell surface glycoconjugates for their activity.