Using yeast two-hybrid screening, we have identified a mouse Postsynaptic density 95/Discs large/Zona occludens-1 (PDZ) protein that interacts with activin type II receptors (ActRIIs). We named the protein activin receptor-interacting protein 2 (ARIP2). ARIP2 was found to have one PDZ domain in the NH 2 -terminal region and interact specifically with ActRIIs among the receptors for the transforming growth factor ␤ family by the PDZ domain. Interestingly, overexpression of ARIP2 enhances endocytosis of ActRIIs and reduces activin-induced transcription in Chinese hamster ovary K1 cells. In addition, immunofluorescence co-localization studies indicated the direct involvement of ARIP2 in the intracellular translocation of ActRIIs by PDZ domain-mediated interaction. Moreover, we have identified that the COOH-terminal region of ARIP2 interacts with Ralbinding protein 1 (RalBP1). RalBP1 is a potential effector protein of small GTP-binding protein Ral and regulates endocytosis of epidermal growth factor and insulin receptors. The studies using deletion mutants of RalBP1 and constitutively GTP and GDP binding forms of Ral indicate that ARIP2 regulates endocytosis of ActRIIs through the Ral/RalBP1-dependent pathway, and the GDP-GTP exchange of Ral is critical for this regulation.Activin, a member of the TGF-␤ 1 superfamily, has a broad range of physiological activities including hematopoiesis, bone morphogenesis, neurogenesis, and hormone action (1-5). These various actions on cell proliferation, differentiation, and apoptosis are dependent upon target cells. Activin transduces its signal via heteromeric complexes composed of two different serine/threonine kinase receptors, termed type I and type II (6 -8). Upon ligand binding, the type II receptor transphosphorylates and activates the type I receptor kinase at the membrane region. Then, the type I receptor cytoplasmic domain interacts with intracellular signaling molecules, Smads, which regulate transcription of selected genes in a cell-specific manner (9, 10).In the current model, the functions of the ActRIIs are limited to ligand binding, type I receptor recruitment, and transphosphorylation. However, it is speculated that there are specific roles of the ActRIIs in activin signaling. For example, the serine/threonine kinase domains of ActRIIs are constitutively activated, and ActRIIs exist as homooligomers (probably homodimers) even in the absence of ligands (11,12). These data suggest the existence of a strict monitoring mechanism for type II receptor kinase activity. Furthermore, multiple forms of ActRIIs exist; two subtypes of activin type II receptors, ActRIIA and ActRIIB, each of which is encoded by individual genes, are known (13,14). Several alternative splicing variants have also been found in each subtype (14 -16). For example, activin type IIA-N receptor, a splicing product of ActRIIA, is specifically expressed in neural cells and is thought to mediate neuronalspecific activin action. In a previous report, we noted the identification of a PDZ protein called a...

Abstract. We studied developmental reproduction of male golden hamsters (Mesocricetus auratus) by determining hormone secretion, observing morphological changes including distribution of immunoexpression of inhibin α subunit, 3β-hydroxysteroid dehydrogenase (HSD), and investigating sperm head count from birth to adulthood. Immunoexpression of inhibin α subunit was found in interstitial cells of not well organized testes of neonatal animals, and positive staining was also found in Sertoli cells in developing animals. However, the intensity of immunostaining in Sertoli cells varied when the spematogenic cycle of the seminiferous epithelium began. Testosterone levels in the plasma and testicular contents of testosterone and immunoreactive (ir)-inhibin increased from around 25 days of age, about 10 days before the presence of the first sperm in the testis. On the other hand, plasma concentrations of ir-inhibin progressively increased from birth to 10 days of age, when it reached peak levels. A reciprocal pattern of change between plasma concentrations of ir-inhibin and FSH was found from 10 days of age. Plasma concentrations and pituitary contents of LH increased from 15 days of age and reached adult levels about 40 days of age. The present results suggest that inhibin is an important factor in the regulation of FSH secretion even in infant male golden hamsters, and the regulated FSH may control the increasing the number of germ cells. Inhibin might be not the only endocrine factor as the regulator of FSH secretion, but also a paracrine or autocrine factor which is involved in spermatogenesis in the golden hamster. Key words: Inhibin, Hamster, Development, Testis (J. Reprod. Dev. 48: [343][344][345][346][347][348][349][350][351][352][353] 2002) arly 20 th century experiments, which were the initial proposal for the existence of inhibin, showed that destruction of the germinal epithelium by irradiation caused hyperfunction in the absence of atrophy of the second sex glands [1], and that injection of aqueous testicular extracts to castrated rats prevented the appearance of enlarged "castration cells" in the pituitary [2,3]. These