Copyright © 1997, American Society for Microbiology Localization of a C-Terminal Region of l2 Protein in Reovirus Cores

The 144-kDa l2 protein is a structural component of mammalian reovirus particles and contains the guanylyltransferase activity involved in adding 5 * caps to reovirus mRNAs. After incubation of reovirus T3D core particles at 52°C, the l2 protein became sensitive to partial protease degradation. Sequential treatments with heat and chymotrypsin caused degradation of a C-terminal portion of l2, leaving a 120K core-associated fragment. The four other proteins in cores—l1, l3, m2, and s2—were not affected by the treatment. Purified cores with cleaved l2 were subjected to transmission cryoelectron microscopy and image reconstruction. Reconstruction analysis demonstrated that a distinctive outer region of l2 was missing from the modified cores. The degraded region of l2 corresponded to the one that contacts the base of the s1 protein fiber in reovirus virions and infectious subvirion particles, suggesting that the s1-binding region of l2 is near its C terminus. Cores with cleaved l2 were shown to retain all activities required to transcribe and cap reovirus mRNAs, indicating that the C-terminal region of l2 is dispensable for those functions. Much remains to be learned about the relationship between reovirus protein sequences and the structural details of reovi-rus particles, such as those that are observed following trans-mission cryoelectron microscopy (cryoTEM) and three-dimen