| Abstract: | Synthesis and turnover of the main structural protein (P73) of intracisternal A-particles were studied in mouse neuroblastoma cells in tissue culture. Triton X-100:EDTA-insoluble pellets containing 95% of the A-particle antigen in the cells were prepared and analyzed by electrophoresis in Na dodecyl-SO4-minus polyacrylamide gels. A 73,000 molecular weight component was prominent in pellets from three lines of neuroblastoma which contain numerous A-particles and this component was identified as the A-particle structural protein P73. It was absent in pellets prepared from cells which do not contain A-particles. Incorporation of labeled amino acids into P73 represented approximately 1.2% of total cell incorporation and this proportion did not change when the cell growth changed from log phase to stationary phase. Label appeared P73 within 2 min after radioactive amino acids were added to the medium. Pulse-chase and inhibitor studies confirmed antigenic measurements in demonstrating that the pool of P73 not assembled into A-particles was small. Turnover studies showed that P73 gained and lost label more rapidly than the average cell protein. In one cell line which was thoroughly characterized, approximately 60% of the main A-particle protein was estimated to turn over in a 24-hour period. Although the cells released approximately 10% of the proteins synthesized into the culture fluid, A-particle protein did not appear to be released. Analysis of culture fluid failed to reveal A-particles, soluble A-particle proteins, or A-particle antigen. It appears, therefore, that the particles are relatively rapidly synthesized and degraded, and that turnover occurs entirely intracellularly. |
