Co-translational trimerization of the reovirus cell attachment protein

The reovirus cell attachment protein, sigma1, is a trimer with a 'lollipop' structure. Recent findings indicate that the N-terminal fibrous tail and the C-terminal globular head each possess a distinct trimerization domain. The region responsible for N-terminal trimerization (formation of a triple alpha-helical coiled-coil) is located at the N-terminal one-third of sigma1. In this study, we investigated the temporality and ATP requirement of this trimerization event in the context of sigma1 biogenesis. In vitro co-synthesis of the full-length (FL) and a C-terminally truncated (d44) sigma1 protein revealed a preference for homotrimer over heterotrimer formation, suggesting that assembly at the N-terminus occurs co-translationally. This was corroborated by the observation that polysome-associated sigma1 chains were trimeric as well as monomeric. Truncated proteins (d234 and d294) with C-terminal deletions exceeding half the length of sigma1 were found to trimerize post-translationally. This trimerization did not require ATP since it proceeded normally in the presence of apyrase. In contrast, formation of stable FL sigma1 trimers was inhibited by apyrase treatment. Collectively, our data suggest that assembly of nascent sigma1 chains at the N-terminus is intrinsically ATP independent, and occurs co-translationally when the ribosomes have traversed past the midpoint of the mRNA.