S100B and S100A1 proteins in bovine retina: Their calcium-dependent stimulation of a membrane-bound guanylate cyclase activity as investigated by ultracytochemistry

The Ca2+-binding proteins of the EF-hand type, S100B and S100A1, were detec ted in the outer segment of bovine retina photoreceptors where they are loc alized to disc membranes, as investigated by immunofluorescence and immunog old cytochemistry. S100B and S100A1 stimulate a membrane-bound guanylate cy clase activity associated with photoreceptor disc membranes in dark-adapted retina in a Ca2+-dependent manner, although with different Ca2+ requiremen ts, as investigated by an ultracytochemical approach. Other retinal cell ty pes express S100B and S100A1 as well. S100B is detected in the outer limiti ng membrane, fine cell processes in the outer nuclear layer and the outer p lexiform layer, cell bodies in the inner nuclear layer and the ganglion cel l layer, and the inner limiting membrane, whereas S100A1 has a more discret e distribution. S100B and S100A1 also stimulate a membrane-bound guanylate cyclase activity in photoreceptor cell bodies and Muller cells, but their e ffect appears independent of the light- or dark-adapted state of the retina and is observed at relatively high Ca2+ concentrations. These data represe nt the ultrastructural counterpart of recent biochemical observations impli cating S100B and, possibly, S100A1 in the Ca2+-dependent stimulation of a p hotoreceptor membrane-bound guanylate cyclase activity [T. Duda, R. M. Gora czniak and R. K. Sharma (1996) Molecular characterization of S100A1-S1000B protein in retina and its activation mechanism of bovine photoreceptor guan ylate cyclast. Biochemistry 35, 6263-6266; A. Margulis, N. Pozdnyakov and A . Sitaramayya (1996) Activation of bovine photoreceptor guanylate cyclast b y S100 proteins. Biochem. Biophys. Res. Commun. 218, 243-247]. Our data suggest that at least S100B may take part in the regulation of a m embrane-bound guanylate cyclase-based signalling pathway in both photorecep tors and Muller cells. (C) 1999 IBRO. Published by Elsevier Science Ltd.