| Research Abstract |
Activins are members of a large family of transforming growth factors beta, and has various functions, including stimulation of hematopoiesis, paracrine regulation of ovarian and testicular function, modulation of nerve cell differentiation and induction of mesodermal tissues in Xenopus embryo. To understand the expression mechanism for diverse actions of activin, we investigated the molecular mechanism of activin signal transduction and obtained the following findings.
(1) We have recently isolated the activin receptor from the mouse embryonal carcinoma (EC) cell line P19, and found that the activin receptor is a transmembrane serine/threonine/tyrosine protein kinase. In this study, we isolated a mouse activin receptor (type II) protein from COS cells transiently expressed the activin receptor (type II) DNA by the same affinity chromatography as from ECP19 cells. The purified receptor protein was also found to phosphorylate both itself and substrate proteins on serine, threonine and ty
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rosine residues. To date, no transmembrane-type kinase with dual kinase activity has been reported. Accordingly, we may propose that signal transduction of activin employs a novel pathway via a new class of cellular receptor.
(2) We examined the stoichiometric interaction between activin and follistatin (activin-binding protein) by FPLC gel filtration, and found that the molar ratio of follistatin to activin was 2 : 1. This ratio was also confirmed by testing the stimulating or inhibiting effect of the activin-follistatin mixtures on FSH-release by cultured pituitary cells. These results indicate that two moles of follistatin is necessary for complete inhibition of the activity of one mole activin. Furthermore, we purified six molecular forms of follistatin from porcine ovaries. Protein chemical analysis revealed that the structural differences among the six forms were caused by truncation of the C-terminal region and/or the presence of carbohydrate chains. All six molecular species were found to have the same activin binding activity, By contrast, the C-terminal truncated form showed much higher affinity for tha rat granulosa cell surface than the C-terminal extended forms. These results suggest that cell-associated follstatin plays an important role in controlling the various actions of activin in a paracrine or autocrine manner. Less
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