A 14-kDa protein was identified as a major component of mature oocytes of the leech Theromyzon tessulutum. This protein was, like vitellin, detected in the content of yolk granules and was purified by gel-permeation and ion-exchange chromatography. The yolk protein corresponded to an ironbinding protein which exists in a monomeric unglycosylated form and had no similarities to vitellin. However, a strong resemblance between this protein and sipunculid hemerythrin, a non-heme ironbinding protein, was observed on the basis of its characteristics including molecular mass, iron content, ultraviolet/visible spectrum, amino acid composition and N-terminal sequence. These similarities with hemerythrin and the accumulation of the protein in the oocyte justify the name ovohemerythrin given to the molecule.A coelomic-fluid protein immunologically related to ovohemerythrin was detected in vitellogenic animals. The protein was purified with the chromatographic procedure used to isolate ovohemerythrin from oocytes and was found to be similar to the oocyte protein. This circulating qvohemerythrin was present in large amounts in the coelomic fluid while gametogenesis is in progress, i. e. after the third and last blood meal of the animal (stage 3), except at the time of oocyte enlargement when its concentration decreases dramatically. However, in contrast to vitellogenin, which is detected specifically in the coelomic fluid of leeches at stage 3, circulating ovohemerythrin is also observed after the first (stage 1) and second (stage 2) blood meal. This observation suggests a more complex function for ovohemerythrin than being merely a yolk nutrient for the embryo. .In most oviparous animals, oogenesis is characterized by an abundant accumulation of yolk proteins which provide the nutritive materials needed for embryogenesis. The major eggyolk protein found in these animals is vitellin. Vitellin is a high-molecular-mass lipoglycoprotein generally composed of multiple subunits of unequal size and comprises more than 50% of the buffer-soluble proteins in the egg [l, 21. This yolk component is derived from a precursor, vitellogenin, which, depending upon the species, is secreted into the blood stream, the hemolymph or the coelomic fluid and is sequestered in the oocytes by receptor-mediated uptake. Vitellogenin synthesis takes place in the liver in vertebrates [3], the fat body in most insects [4], coelomocytes in nereid annelids [5, 61 and the intestine in nematodes [7] and echinoderms where vitellogenin production also occurs in the gonads of both sexes [8]. Extensive biochemical and physiological information has been obtained concerning the major egg-yolk proteins in a wide variety of organisms and it appears that the vitellogenins are
Tlzeromyzon tessulatum vitellin was identified as a lipoglycoprotein of 490 kDa. The insolubility of this molecule in low-ionic-strength media was used to extract it from the ovaries. Antiserum prepared against vitellin was shown to react with a coelomic fluid component of 520 kDa. This vitellin precursor, or vitellogenin, was purified by gel permeation and ion-exchange column chromatography. These two lipoglycoproteins were characterized by amino acid, carbohydrate and lipid analysis and subunit composition. In spite of differences in terms of native molecular mass, solubility and isoelectric point, the lipoglycoproteins isolated from the coelomic fluid and the ovary were similar in their subunit components (a single polypeptide of 165 kDa) and in their amino acid and carbohydrate compositions. However, vitellogenin was found to be more highly lipidated (31.8% by mass) than vitellin (24% by mass) and lipid analysis indicated a higher amount of sterols and phospholipids in vitellogenin. From these data, we conclude that vitellogenin and vitellin are probably dimers of two identical subunit polypeptides plus lipid and that, after vitellogenin is sequestered in the oocyte, part of its lipid component is stripped from the molecule to give vitellin. Furthermore, electrophoretic analysis seems to indicate that vitellogenin synthesis and secretion is initiated following the third and last blood meal of the animal but that vitellogenin significantly accumulates in the coelomic fluid before being incorporated in the oocytes suggesting a complex mode of vitellogenesis regulation.The production of egg yolk involves a massive synthesis of specific proteins which is under hormonal control. These proteins, termed vitellogenins, are generally synthesized externally to the site of accumulation. Depending upon the species, vitellogenins are secreted in the bloodstream or in the hemolymph, taken up by the developing oocytes and deposited as vitellins, the major yolk proteins [l, 21. Thus, in addition to its physiological importance for reproduction, vitellogenesis also provides a good system for the study of molecular mechanisms and hormonal regulation of egg maturation.Yolk proteins have been extensively investigated in oviparous vertebrates [I] and in insects [3, 41. In annelids, oocyte vitellin has been purified from nereid polychaetes [5, 61 and it has been shown that oocyte differentiation depends upon the incorporation of vitellogenin [7, 81. On the other hand, little is known for the two groups of clitellates: oligochaetes [9, 101 and hirudinae [I I]. In clitellates, eggs are generally of a small size and laid within a cocoon secreted by the clitellum and abundantly provided with reserve substances from which the embryo will develop, therefore making an accumulation of
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