Exploration of the extracellular space by a large-scale secretion screen in the early Xenopus embryo

Secreted proteins play a crucial role in intercellular communication during embryogenesis and in the adult. We recently described a novel method, designated as secretion cloning, that allows identifying extracellular proteins exclusively based on their ability to be secreted by transfected cells. In this paper, we present the results of a large-scale screening of more than 90,000 clones from three cDNA expression libraries constructed from early Xenopus embryos. Of 170 sequenced clones, 65 appeared to encode secreted proteins; 26 clones (40%) were identical to previously known Xenopus genes, 25 clones (38%) were homologous to other genes identified in various organisms and 14 clones (22%) were novel. Apart from these bona fide secreted proteins, we also isolated lysosomal or other secretory pathway proteins and some cytoplasmic proteins commonly found in body fluids. Among the novel secreted proteins were two putative growth factors of the Granulin family, termed xGra1 and xGra2; they are structurally similar to EGF and TGFalpha and show a spotted expression pattern in the epidermis. Another secreted protein, designated xSOUL, belongs to the family of heme-binding proteins and exhibits distinct expression in the early brain. A third protein, termed Xystatin, is related to cysteine proteinase inhibitors. Our results indicate that secretion cloning is an effective and generally useful tool for the unbiased isolation of secreted proteins.