LOCALIZATION OF A C-TERMINAL REGION OF LAMBDA-2 PROTEIN IN REOVIRUS CORES

The 144-kDa lambda 2 protein is a structural component of mammalian re ovirus particles and contains the guanylyltransferase activity involve d in adding 5' caps to reovirus mRNAs. After incubation of reovirus T3 D core particles at 52 degrees C, the lambda 2 protein became sensitiv e to partial protease degradation, Sequential treatments with heat and chymotrypsin caused degradation of a C-terminal portion of lambda 2, leaving a 120K core-associated fragment, The four other proteins in co res-lambda 1, lambda 3, mu 2, and sigma 2-were not affected by the tre atment, Purified cores with cleaved lambda 2 were subjected to transmi ssion cryoelectron microscopy and image reconstruction, Reconstruction analysis demonstrated that a distinctive outer region of lambda 2 was missing from the modified cores. The degraded region of lambda 2 corr esponded to the one that contacts the base of the sigma 1 protein fibe r in reovirus virions and infectious subvirion particles, suggesting t hat the sigma 1-binding region of lambda 2 is near its C terminus. Cor es with cleaved lambda 2 were shown to retain all activities required to transcribe and cap reovirus mRNAs, indicating that the C-terminal r egion of lambda 2 is dispensable for those functions.