Selectivity of protein sequestration by vitellogenic oocytes of the rainbow trout, Salmo gairdneri

The selectivity of protein sequestration by vitellogenic oocytes of the rainbow trout, Salmo gairdneri , was investigated by comparing the rates of uptake of radiolabelled vitellogenin (VTG) and bovine serum albumin (BSA). Fish were injected simultaneously with equal amounts of both 3 H‐VTG and 14 C‐BSA and sampled subsequently at varying time invervals to assess the rates of uptake of these proteins into the developing oocytes. Both labelled proteins were taken up rapidly into the blood after their injection into the peritoneal cavity. Once in the blood, they became available for sequestration by the vitellogenic oocytes. There was both rapid and selective uptake of 3 H‐VTG; within 72 hours half of the 3 H‐VTG injected was present within the ovary, whereas only 2.5% of the 14 C‐BSA was similarly located. Once within the oocyte, 3 H‐VTG was specifically and rapidly processed to yield its constituent yolk proteins, which included 3 H‐lipovitellin. In contrast, after 72 hours 14 C‐BSA was present in the oocyte as the native, intact protein. However, after 13 days, whereas the yolk proteins had undergone no further processing, remaining stable within the oocyte, much of the BSA within the ovary had been degraded to an array of smaller peptides. Furthermore, after 26 days, whereas all of the 3 H‐VTG endocytosed remained within the oocytes as yolk protein, at least half of the BSA taken up initially had been exocytosed. Calculation suggested that 3 H‐VTG was sequestered about 60 times more rapidly than 14 C‐BSA. Other tissue did not incorporate 3 H‐VTG to any significant degree. These results suggest that VTG was sequestered into vitellogenic oocytes by receptor‐mediated endocytosis, whereas BSA, along with other extraneous proteins, was taken up adventitiously.