Protein transport: A selective membrane mechanism

Proteins are selectively sequestered by a number of cell types. However, only in oocytes is the process sufficiently aggravated and specific to be readily studied. In these cells certain serum proteins are taken up in proportions different from those found in the serum. In vitro incubations of hormonally stimulated and synchronous mosquito oocytes show that the only protein capable of initiating the transport process is the female specific yolk protein. Heterologous proteins such as IgG, bovine serum albumin, cytochrome C, and ferritin are inactive. The female specific protein is a phosphoglycolipoprotein. It is synthesized in the fat body, a liver analog in the insect, and passed into the serum before being transported into the oocytes. Preliminary kinetic analysis shows the uptake process to be specific with an apparent Km of about 10(-7) M. Glycolytic inhibitors stop protein uptake. The receptor-mediated binding steps in the transport process are most easily studied in the chicken because of the enormous amount of oocyte membrane available from a given oocyte and because up to 1 gm of protein is normally transported per day per oocyte. IgG and the hen specific phosvitin lipovitellin are two of the physiologically important proteins that are transported intact into the chicken oocytes. The uptake appears selective as shown by studies with iodinated proteins. Ferritin conjugated to IgG is shown by electron microscopy to bind to isolated plasma membranes only where coated pits have formed, whereas ferritin alone is not seen localized on any membrane surface. These very specialized regions of the membrane are similar to micropinocytotic pits but, in addition, possess on their cytoplasmic side dense ridges that form the coat. Transport involves binding to the coated pits, the pinching off of the pits, and the subsequent movement of the coated vesicles in the cytoplasm.