Characterization of an alpha-macroglobulin-like glycoprotein isolated fromthe plasma of the soft tick Ornithodoros moubata

We report the identification of the first representative of the alpha-2-mac roglobulin family identified in terrestrial invertebrates. An abundant acidic glycoprotein was isolated from the plasma of the soft ti ck Ornithodoros moubata. Its molecular mass is about 420 kDa in the native state, whereas in SDS/PAGE it migrates as one band of 190 kDa under nonredu cing conditions and a band of 92 kDa when reduced. Chemical deglycosylation reveals that it is composed of two different subunits, designated A and B. The N-terminal amino-acid sequence of subunit A is similar to the N-terminu s of invertebrate alpha-2-macroglobulin. Sequence analysis of several inter nal peptides confirms that the tick protein belongs to the alpha-2-macroglo bulin family, and the protein is therefore referred to as tick ol-macroglob ulin (TAM). Functional analyses strengthen this assignment. TAM inhibits tr ypsin and thermolysin cleavage of the high-molecular-weight substrate azoco ll in a manner similar to that of bovine alpha-2-macroglobulin. This effect is abolished by pretreatment of TAM with methylamine. In the presence of T AM, trypsin is protected against active-site inhibition by soybean trypsin inhibitor. We cloned and sequenced a PCR product containing sequences from both subuni ts and spanning the N-terminus of subunit B and the putative 'bait region' (a segment of alpha-2-macroglobulin which serves as target for various prot eases). This indicates that the two subunits are generated from a precursor polypeptide by posttranslational processing.