Development of MEKA (phosducin), Gβ, Gγ and S-antigen in the rat pineal gland and retina

Abstract

Pinealocytes and retinal photoreceptor cells contain an unusual cytoplasmic complex composed of the Gβγ dimer of GTP-binding regulatory proteins (G-proteins) tightly bound to an acidic 33 kDa phosphoprotein termed MEKA or phosducin; MEKA is a substrate of cyclic AMP-dependent protein kinase. This study characterized the developmental appearance of these and two related proteins, Gγ and S-antigen, in pineal and retinal tissue. MEKA was absent in the pineal gland prior to birth, at a time when it was possible to detect Gβ in pineal cytoplasm, indicating that the appearance of Gβ in the cytoplasm precedes that of MEKA and does not appear to require the presence of MEKA. The absence of MEKA at this time indicates that the cyclic AMP stimulation of pineal serotonin N-acetyltransferase activity is not mediated by MEKA, which has been considered as a possible role of MEKA. After postnatal day 7, pineal MEKA and cytoplasmic Gβ increased in a parallel manner, with peak values occurring at about postnatal day 21. Thereafter, both proteins in the pineal gland decreased in a parallel fashion to 10 and 35% of their peak values, respectively; in contrast, the cytoplasmic protein S-antigen and membrane associated Gβ remained at maximal levels after this time. Whereas both MEKA and Gβ decreased late in development in the pineal gland, these proteins either increased or remained constant in the retina. These tissue-specific patterns were found to differ from those of another cytosolic protein found exclusively in the pineal gland and retina, S-antigen, which remained constant after 21 in the pineal gland but decreased in the retina late in life.