Degradation of maternal factors during preimplantation embryonic development.

During oogenesis, oocytes accumulate a large set of proteins derived from the maternal genome. These proteins, known as maternal proteins, are not only required for oocyte maturation and fertilization, but also implicated in subsequent embryonic development. However, most maternal proteins are degraded and their amino acid components are utilized for newly synthesized proteins from the embryonic genome. This process is known as the oocyte-to-embryo transition; because it occurs over a short period, mechanisms involving massive degradation of maternal proteins have been proposed. Intracellular protein degradation mechanisms can be broadly classified into two types. The first is the ubiquitin-proteasome system, a highly selective pathway in which ubiquitylated proteins are degraded by proteasomes. The second mechanism is autophagy, which involves lysosome-mediated degradation of cytoplasmic components. In this review, we describe recent advances in the understanding of autophagy, focusing on its role in early embryonic development.