Methods for the expression in Pichia pastoris and purification of the human activin receptor type I and II extracellular domains (ARIa/ARIb-ECDs, ARIIA/ARIIB-ECDs) are described. Key experimental aspects are also documented of the vector transformation methodology and the binding characteristics of these ECDs with activin A and inhibin. The cDNA constructs for these ECDs contained a C-terminal His(6)-tag with either the native signal (N) or the yeast alpha mating factor (alpha MF) sequence and were introduced into the pPICZ expression vector either as a single-copy or as a four-copy expression cassette. Hyper-resistant transformants (zeo(R): 500 mu g/mL) generated from the cassette containing a single copy of the expression vector gave the stronger signal intensity with a DNA dot-blot screening assay. These transformants also produced higher quantities of the corresponding recombinant protein compared to transformants using the four-copy cassette vector. All receptor-ECD proteins expressed were found to be heterogeneously glycosylated, whereby the ARIIA-ECD and ARIIB-ECD had undergone two Asn-linked glycosylation events and the ARIb-ECD a single event. By SDS-PAGE, the de-glycosylated proteins migrated larger than the expected core size, indicating that they may have undergone O-linked glycosylation. Biacore-based procedures with the glycosylated and de-glycosylated ARIIA-ECD were employed to determine the kinetic and equilibrium binding parameters for the interaction with activin A and inhibin. The glycosylated ARIIA-ECD bound to activin A with a K-D Of 11.9 nM and inhibin with a K-D of 21.1 nM. Although glycosylation of ARIIA-ECD was not strictly required for high affinity interactions with activin A or inhibin, it markedly improved the overall stability of the ARIIA-ECD. (c) 2005 Elsevier Inc. All rights reserved.