C-terminal trimerization, but not N-terminal trimerization, of the reovirus cell attachment protein is a posttranslational and Hsp70/ATP-dependent process

The C-terminal globular head of the lollipop-shaped σ1 protein of reovirus is responsible for interaction with the host cell receptor. Like the N-terminal fibrous tail, it has its own trimerization domain. Whereas N-terminal trimerization (formation of a triple α-helical coiled coil) occurs at the level of polysomes (i.e. cotranslationally) and is ATP-independent, C-terminal trimerization is a posttranslational event that requires ATP. Coprecipitation experiments using anti-Hsp70 antibodies and truncated σ1 proteins synthesized in vitro revealed that only regions downstream of the N-terminal α-helical coiled coil were associated with Hsp70. Hsp70 was also found to be associated with nascent σ1 chains on polysomes as well as with immature postribosomal σ1 trimers (hydralike intermediates with assembled N termini and unassembled C termini). These latter structures were true intermediates in the σ1 biogenetic pathway since they could be chased into mature σ1 trimers with the release of Hsp70. Thus, unlike N-terminal trimerization, C-terminal trimerization is Hsp70- and ATP-dependent. The involvement of two mechanistically distinct oligomerization events for the same molecule, one cotranslational and one posttranslational, may represent a common approach to the generation of oligomeric proteins in the cytosol.