Isolation andsequence ofacDNAencoding theprecursor ofa bombesinlike peptide frombrain andearly embryos of Xenopuslaevis

A cDNAencoding theprecursor ofabom- besinlike peptide wasisolated frombrain ofXenopus laevis. The predicted endproduct resembles neuromedin B,whichwas originafly isolated frommammalian spinal cord. ThemRNA forthis precursor wasalso present ingastrointestinal tract and inovaries. Moreover, itcould bedetected inearly embryos (stage 2andstage 10)ofX.laevis. These findigs suggest novel roles forpeptides ofthebombesin family inoocyte maturation andearly amphibian development. Bombesin andranatensin arerelated peptides originally iso- lated fromskinofBombinabombina andRanapipiens, respectively (1,2).Several peptides belonging tothis family havesince beendetected inskin ofother frog species (3, 4). As inother instances, homologues ofthese amphibian peptides werelater found tobepresent inmammalian endocrine and neural cells also. Gastrin-releasing peptide (GRP)andits carboxyl-terminal decapeptide, formerly called neuromedin C,arebothwidely distributed inthenervous system and gastrointestinal tract aswell asinnormal andneoplastic lung (5-9). Thesepeptides stimulate therelease ofhormones and enzymes (reviewed inref. 3)andalso actaspotent mitogens oncertain normal andtumorcells (10-12). Neuromedin B, another memberofthefamily ofbombesinlike peptides, was originally isolated fromporcine spinal cord(13, 14). Itisalso present inmanyparts ofthebrain andthegastrointestinal tract, yetitsdistribution differs fromthatofGRPandits smaller fragments (6). Bombesin andrelated peptides bind to cellular receptors which belong totheclass that iscoupled to guanine nucleotide-binding proteins (15, 16). Present evidence indicates that inmammals atleast twotypes ofreceptors with different affinities forGRPandneuromedin Bexist (17-19). Byusing molecular cloning techniques, thestructures of theprecursors forseveral ofthesepeptides havebeen deduced (20-24). Inparticular, thecDNAsofthebombesin precursors fromtwospecies ofBombina, B.variegata andB. orientalis, haverecently beenanalyzed (23, 24). Thepre- dicted precursor polypeptides contain asingle copyofthe biologically active endproduct. Using thecDNAcloned from skin ofB.variegata, wehavenowisolated acDNAencoding ahomologous precursor fromalibrary prepared frombrain ofXenopus laevis.t Thecorresponding mRNA-is also present inother tissues, anditisalready expressed inearly embryos ofthis frog.