Yolk proteins from insect eggs: Structure, biosynthesis and programmed degradation during embryogenesis

Since insect vitellogenesis is a heterosynthetic process, yolk protein follows a complicated fate. This includes: hormonally stimulated gene activity, secretion into the blood, specific recognition and uptake by oocytes, packaging into yolk granules, and proteolysis in developing eggs. During the process, proteins are modified co- or post-translationally. Each of these steps has been intensively studied in insects. This offers a basis for more exciting development not only in the field of insect physiology, but also in more basic or fundamental areas of biochemistry and cellular biology. Bombyx acid cysteine proteinase is a case in point. It is now evident that proteolytic reactions play a key role in the control of many physiological functions. Thus the control mechanisms involved in the proteolytic processes require attention and proteinases have been purified and characterized. In many cases, however, it is difficult to analyze the physiological substrates of them, and difficult to know the regulatory mechanism in vivo. Insect yolk protein systems provide a suitable system for studying regulation of cellular proteinases, since yolk proteins (substrate of proteinases) are purified and characterized. This review summarizes recent progress in understanding the structure, biosynthesis and programmed degradation of yolk proteins in insects.