Identification in Xenopus Laevis of a Class of Oocyte-specific Proteins Bound to Messenger RNA

Proteins associated with poly(A)-rich RNA in Xenopus laevis oocytes have been identified in immature ovary homogenates using sucrose gradient centrifugation, oligo(dT)-cellulose fractionation and analysis by sodium dodecyl sulphate/polyacrylamide gel electrophoresis and two-dimensional acid/urea/sodium dodecyl sulphate gel electrophoresis. At least eight proteins are associated with ribonucleoprotein complexes and have the following properties. 1. Four major proteins, mRNP1-4, cosediment with poly(A)-rich RNA at 40--200 S, bind to oligo(dT)-cellulose and are oocyte-specific. These are very basic proteins with molecular weights of about 50 X 10(3), 52 X 10(3), 56 X 10(3), 59 X 10(3). 2. Two further proteins, mRNP5,6, molecular weights 75 X 10(3) and about 100 X 10(3), also co-isolate with poly(A)-rich RNA mRNP7 (Mr 16 X 10(3)) cosediments with poly(A)-rich RNA but fails to bind to oligo(dT)-cellulose. mRNP8 (Mr 22 X 10(3)) binds to oligo(dT)-cellulose but sediments more slowly than the bulk of poly(A)-rich ribunucleoprotein, at 30--70 S. 3. mRNP1-4 are present in immature oocytes but only mRNP3-4 are found in full-grown oocytes, while none of them could be detected in Xenopus liver or reticulocytes. These proteins are largely cytoplasmic, associated with free mRNP particles, and differ in molecular weight from proteins isolated from polysomal mRNPs in somatic tissues. 4. mRNP1-4 are very abundant in immature oocytes; sedimentation properties are consistent with a protein:RNA ratio of 4:1 (w/w). The sedimentation constant of mRNP particles is resistant to concentrations of ribonuclease A which degrade ribosomes, although mRNA isolated from these ribonuclease-treated particles appears to be partially degraded. The role of ribonucleoprotein complexes in the stability and storage of mRNA during oogenesis is discussed.