RESUMEN
The egg envelope of most animal eggs is modified following fertilization, resulting in the prevention of polyspermy and hardening of the egg envelope. In frogs and mammals a prominent feature of envelope modification is N-terminal proteolysis of the envelope glycoprotein ZPA. We have purified the ZPA protease from Xenopus laevis eggs and characterized it as a zinc metalloprotease. Proteolysis of isolated egg envelopes by the isolated protease resulted in envelope hardening. The N-terminal peptide fragment of ZPA remained disulfide bond linked to the ZPA glycoprotein moiety following proteolysis. We propose a mechanism for egg envelope hardening involving ZPA proteolysis by an egg metalloprotease as a triggering event followed by induction of global conformational changes in egg envelope glycoproteins.
Asunto(s)
Proteínas del Huevo/química , Glicoproteínas de Membrana/química , Receptores de Superficie Celular/química , Zona Pelúcida/química , Animales , Anticuerpos/química , Western Blotting , Clonación Molecular , ADN Complementario/metabolismo , Electroforesis en Gel de Poliacrilamida , Inhibidores Enzimáticos/farmacología , Femenino , Fertilización , Masculino , Metaloproteasas/metabolismo , Óvulo/metabolismo , Péptido Hidrolasas/química , Inhibidores de Proteasas/farmacología , Estructura Terciaria de Proteína , Proteínas/química , Espermatozoides/metabolismo , Temperatura , Xenopus laevis , Zona Pelúcida/metabolismo , Glicoproteínas de la Zona PelúcidaRESUMEN
A Xenopus laevis egg cortical granule, calcium-dependent, galactosyl-specific lectin participates in forming the fertilization layer of the egg envelope and functions in establishing a block to polyspermy. We report the cDNA cloning of the lectin, expression of the cortical granule lectin gene during oogenesis and early development, and identification of a new family of lectins. The translated cDNA for the cortical granule lectin had a signal peptide, a structural sequence of 298 amino acids, a molecular weight of 32.7 K, contained consensus sequence sites for N-glycosylation and a fibrinogen domain. The lectin cDNA was expressed during early stages of oogenesis. Lectin glycoprotein levels were constant during development with 2/3 of the lectin associated with the extracellular perivitelline space and the egg/embryo fertilization envelope. Lectin mRNA levels were from 100- to 1000-fold greater in ovary than in other adult tissues. The lectin had no sequence homology to the previously identified lectin families. The lectin had 41-88% amino acid identity with nine translated cDNA sequences from an ascidian, lamprey, frog, mouse, and human. Based on the conserved carbohydrate binding and structural properties of these glycoproteins, we propose a new family of lectins, the eglectin family.