Difference between follistatin isoforms in the inhibition of activin signalling - Activin neutralizing activity of follistatin isoforms is dependent on their affinity for activin

We demonstrate the difference between the follistatin isoforms (FS-288 and FS-315), two activin-binding proteins, in the neutralizing activity for act ivin signalling. Transcriptional reporter assay using 3TP-Lux, an activin-r esponsive reporter construct, showed that the inhibitory effect of FS-288 o n activin-induced transcriptional response is more potent than that of FS-3 15. The potency was not influenced by the presence of heparan sulfates, by which FS, in particular FS-288, associates with cell surfaces at a high aff inity. Furthermore, FS-288 inhibited the binding of activin to its type II receptor more markedly than did FS-315, as evidenced by surface plasmon res onance and affinity cross-linking experiments. Moreover, the Kd of FS-288 a nd FS-315 for activin A was estimated to be 46.5 +/- 0.37 pM and 432 +/- 26 pM, respectively, by surface plasmon resonance experiments. These results indicate that the different potency between the two FS isoforms in the inhi bition of activin activities depends on their affinity for activin A. (C) 2 000 Elsevier Science Inc. All rights reserved.