A dorsoventral asymmetry in the embryonic retina defined by protein conformation ( laminin receptor / retinotopic speciflication / ribosomes )

Content Provider	Semantic Scholar
Author	McCaffery, Peter Drager, Ursula C.
Abstract	In a search for determinants of retinotopic specification we previously identified an antigen in the dorsal embryonic retina as a protein called the 68-kDa laminin receptor. A dorso-ventral asymmetry in a laminin receptor seemed consistent with the known responsiveness of embryonic optic axons to laminin, but there were three peculiar points. (0) The molecular mass of this presumed laminin receptor in immunoblots is not 68 kDa but 43 kDa, and the molecular mass of the protein deduced from the mRNA is only 33 kDa. (it) The antigen does not have the localization expected ofa receptor for the extracellular matrix: the antibodies label mainly a granular cytoplasmic antigen in dorsal retina; an additional sparse cell-surface antigen present on a few cells does not show a dorso-ventral asymmetry. (iM) Despite the pronounced dorsoventral difference seen immunohistochemically, in immunoblots the 43-kDa protein (p40) is evenly distributed throughout the retina. Here we show that (a) native p40 and in vitrotranslated gene product are indistinguishable and their anomalous migration in denaturing gels probably is due to low pI; (it) p40 is bound in a Mg2+-dependent manner to large cytoplasmic complexes that appear to include ribosomes; and (iiM) there is a labile conformational difference in p40 between dorsal and ventral retina: dorsally it is more accessible to proteolysis, suggesting a more open conformation. In conjunction with the recent hypothesis that p40 constitutes a translation initiation factor (D. Auth and G. Brawerman, personal communication), these observations point to a dorso-ventral asymmetry in some aspect of protein translation, which in turn may set up differences in recognition factors on retinal growth cones. The retina projects onto its central target regions in retinotopic maps that are similar in all vertebrates. Most evidence suggests a biochemical basis for the initial steps of the map formation (1). Sperry's chemoaffinity hypothesis postulates a system of two graded chemical determinants, which are orthogonally arranged in the antero-posterior and dorsoventral axes of the retina (2). A long-ongoing search for the molecular mechanism of this process has proven difficult (3), which is not surprising if it turns out to resemble in complexity the axial determination of the early Drosophila embryo. Here the determination of the dorso-ventral axis alone involves the concerted action of proteins encoded by 12 genes, of which the dorsal protein functions as the morphogen, whose nuclear localization is spatially controlled by other factors, and which, in turn, influences the transcription from four genes (4-6). In the developing vertebrate retina one has to assume the existence of at least two separate components: an early one that sets up an asymmetry in the retina as an embryonic field and a later one, expressed on growth-cone surfaces, that executes this asymmetry in the guidance of axons to their targets. In a search for factors involved in retinal determination, we generated two monoclonal antibodies that label very strongly the dorsal part of the embryonic retina in cold-blooded vertebrates, birds, and mammals (7). This dorsal antigen seemed to belong to the early component of factors acting in an embryonic field, as the antigen has a cytoplasmic localization and appears very early in eye development-at the late eye-vesicle stage-and disappears with differentiation of the retina. Upon isolating the cDNA encoding this antigen, however, we were surprised to learn that it is identical to a cell-surface component: a receptor for the extracellular matrix known as the high-affinity or 68-kDa laminin receptor (7-14). Here we present a short biochemical characterization of the protein, and we provide evidence for its interaction with large cytoplasmic complexes that include ribosomes, in which conformation may determine an asymmetrical function in the dorso-ventral axis. It seems now likely that the protein belongs to the early component of factors, possibly playing a role in differential control of protein synthesis.
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